diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..db12181
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,2 @@
+[*.{fs,fsx}]
+max_line_length = 80
\ No newline at end of file
diff --git a/.idea/.idea.Demeton/.idea/watcherTasks.xml b/.idea/.idea.Demeton/.idea/watcherTasks.xml
index a06fc6f..71c6041 100644
--- a/.idea/.idea.Demeton/.idea/watcherTasks.xml
+++ b/.idea/.idea.Demeton/.idea/watcherTasks.xml
@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="ProjectTasksOptions">
-    <TaskOptions isEnabled="false">
+    <TaskOptions isEnabled="true">
       <option name="arguments" value="fantomas &quot;$FilePath$&quot;" />
       <option name="checkSyntaxErrors" value="true" />
       <option name="description" />
diff --git a/Demeton.Tests/Demeton.Tests.fsproj b/Demeton.Tests/Demeton.Tests.fsproj
index 86aaa5d..29e4e46 100644
--- a/Demeton.Tests/Demeton.Tests.fsproj
+++ b/Demeton.Tests/Demeton.Tests.fsproj
@@ -19,6 +19,7 @@
 
     <ItemGroup>
         <RuntimeHostConfigurationOption Include="System.Globalization.Invariant" Value="true"/>
+        <Compile Include="Should.fs" />
         <Compile Include="TestLog.fs"/>
         <Compile Include="TestHelp.fs"/>
         <Compile Include="BetterFsCheckRunner.fs"/>
diff --git a/Demeton.Tests/Should.fs b/Demeton.Tests/Should.fs
new file mode 100644
index 0000000..6293c39
--- /dev/null
+++ b/Demeton.Tests/Should.fs
@@ -0,0 +1,14 @@
+[<RequireQualifiedAccess>]
+module Should
+
+open System
+
+let notBeCalled _ = invalidOp "bug: should not be called"
+let notBeCalled2 _ _ = invalidOp "bug: should not be called"
+let notBeCalled3 _ _ _ = invalidOp "bug: should not be called"
+let notBeCalled4 _ _ _ _ = invalidOp "bug: should not be called"
+
+let fail message = invalidOp message
+
+let reportNotImplemented message =
+    NotImplementedException message |> raise
diff --git a/Demeton.Tests/Srtm/Fetching SRTM tiles/Processing the fetching stack.fs b/Demeton.Tests/Srtm/Fetching SRTM tiles/Processing the fetching stack.fs
index 2ce3b2e..289ccab 100644
--- a/Demeton.Tests/Srtm/Fetching SRTM tiles/Processing the fetching stack.fs	
+++ b/Demeton.Tests/Srtm/Fetching SRTM tiles/Processing the fetching stack.fs	
@@ -37,11 +37,11 @@ let ``Calling processNextCommand on an empty command stack returns the same stat
         |> processNextCommand
             localCacheDir
             srtmDir
-            _noCall
-            _noCall
-            _noCall2
-            _noCall2
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     test <@ resultingState = initialState @>
 
@@ -56,10 +56,10 @@ let ``When a tile does not exist, puts None in the tiles stack`` () =
             localCacheDir
             srtmDir
             (fun _ -> NotExists)
-            _noCall
-            _noCall2
-            _noCall2
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     test <@ resultingState = (initialCommands, None :: initialStackedTiles) @>
 
@@ -81,9 +81,9 @@ let ``When a tile is cached, reads it and puts it into the tiles stack`` () =
             srtmDir
             (fun _ -> Cached)
             (expectReadingOfTile tile)
-            _noCall
-            _noCall2
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     test
         <@
@@ -107,9 +107,9 @@ let ``If reading of a cache tile fails, put error indicator into the stack``
             srtmDir
             (fun _ -> Cached)
             readingOfTileFails
-            _noCall
-            _noCall2
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     test
         <@
@@ -128,10 +128,10 @@ let ``When a level 0 tile is not cached, puts ConvertTileFromHgt command`` () =
             localCacheDir
             srtmDir
             (fun _ -> NotCached)
-            _noCall
-            _noCall2
-            _noCall2
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     test
         <@
@@ -152,10 +152,10 @@ let ``When a level > 0 tile is not cached, fills the command stack with children
             localCacheDir
             srtmDir
             (fun _ -> NotCached)
-            _noCall
-            _noCall2
-            _noCall2
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     let childTiles =
         [| (8, 16); (9, 16); (8, 17); (9, 17) |]
@@ -192,11 +192,11 @@ let ``When convert from HGT command is received`` () =
         |> processNextCommand
             localCacheDir
             srtmDir
-            _noCall
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled2
             convertProducesSomeTile
-            _noCall2
-            _noCall2
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     test
         <@
@@ -221,11 +221,11 @@ let ``Convert to PNG can fail`` () =
         |> processNextCommand
             localCacheDir
             srtmDir
-            _noCall
-            _noCall2
+            Should.notBeCalled
+            Should.notBeCalled2
             convertThatFails
-            _noCall2
-            _noCall2
+            Should.notBeCalled2
+            Should.notBeCalled2
 
     let expectedResultingState =
         (Failure errorMessage :: initialCommands, initialStackedTiles)
@@ -264,9 +264,9 @@ let ``When create from lower tiles command is received and create returns tile``
         |> processNextCommand
             localCacheDir
             srtmDir
-            _noCall
-            _noCall2
-            _noCall
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled
             constructParentTileReturnsSomeTile
             writeTileToCache
 
@@ -311,9 +311,9 @@ let ``When create from lower tiles command is received and create returns None``
         |> processNextCommand
             localCacheDir
             srtmDir
-            _noCall
-            _noCall2
-            _noCall
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled
             constructParentTileReturnsNone
             writeTileToCache
 
@@ -345,8 +345,8 @@ let ``Testing the tail recursion`` () =
             (fun _ -> Cached)
             (fun _ _ -> Ok someTileHeights)
             (fun _ _ _ -> Ok someTileHeights)
-            _noCall2
-            _noCall2
+            Should.notBeCalled2
+            Should.notBeCalled2
             initialState
 
     // there should be no more commands in the stack
@@ -390,10 +390,10 @@ let ``Command stack processor should stop on failure and return the error`` () =
             localCacheDir
             srtmDir
             (fun _ -> NotCached)
-            _noCall2
+            Should.notBeCalled2
             (convertFailsOnSomeCall 4)
-            _noCall2
-            _noCall2
+            Should.notBeCalled2
+            Should.notBeCalled2
             initialState
 
     // there should be an error indicator next in the command stack
diff --git a/Demeton.Tests/Srtm/Saving SRTM tiles to cache.fs b/Demeton.Tests/Srtm/Saving SRTM tiles to cache.fs
index 7308d80..e48ee46 100644
--- a/Demeton.Tests/Srtm/Saving SRTM tiles to cache.fs	
+++ b/Demeton.Tests/Srtm/Saving SRTM tiles to cache.fs	
@@ -51,7 +51,7 @@ let ``Saves the tile into the cache directory`` () =
             cacheDir
             ensureCacheDirectoryExists
             writeAsPngFile
-            _noCall
+            Should.notBeCalled
             tile
             (Some heights)
 
diff --git a/Demeton.Tests/TestHelp.fs b/Demeton.Tests/TestHelp.fs
index 42dbeb2..8e04c34 100644
--- a/Demeton.Tests/TestHelp.fs
+++ b/Demeton.Tests/TestHelp.fs
@@ -4,16 +4,12 @@ open System
 open System.Reflection
 open System.Runtime.InteropServices
 
-let isLinux() = RuntimeInformation.IsOSPlatform(OSPlatform.Linux) 
+let isLinux () =
+    RuntimeInformation.IsOSPlatform(OSPlatform.Linux)
 
 let fail errorMessage =
     raise (Xunit.Sdk.XunitException errorMessage)
 
-let _noCall _ = invalidOp "bug: should not be called"
-let _noCall2 _ _ = invalidOp "bug: should not be called"
-let _noCall3 _ _ _ = invalidOp "bug: should not be called"
-let _noCall4 _ _ _ _ = invalidOp "bug: should not be called"
-
 let isOk result =
     match result with
     | Ok _ -> true
@@ -31,7 +27,7 @@ let isOkValue expectedOkValue result =
 let resultValue result =
     match result with
     | Ok x -> x
-    | Error msg -> 
+    | Error msg ->
         invalidOp (sprintf "The result indicates an error: '%s'." msg)
 
 let isError (result: Result<'T, 'TError>) =
@@ -44,26 +40,30 @@ let isErrorData (errorData: 'TError) (result: Result<'T, 'TError>) =
     | Ok _ -> false
     | Error actualErrorData -> actualErrorData = errorData
 
-let inline (=~=) (x: float) (y: float) = abs (x-y) <  1.E-10
-    
+let inline (=~=) (x: float) (y: float) = abs (x - y) < 1.E-10
+
 type ApproxMeasure =
-    Decimals of int
+    | Decimals of int
     | Percentage of float
 
-let isApproxEqualTo 
-    (controlValue: float) (measure: ApproxMeasure) (actualValue: float) =
+let isApproxEqualTo
+    (controlValue: float)
+    (measure: ApproxMeasure)
+    (actualValue: float)
+    =
     match measure with
-    | Decimals decimals -> 
-        Math.Round(controlValue, decimals) 
-            = Math.Round(actualValue, decimals)
+    | Decimals decimals ->
+        Math.Round(controlValue, decimals) = Math.Round(actualValue, decimals)
     | Percentage percentage ->
         let percentageValue = controlValue * percentage / 100.
         Math.Abs(actualValue - controlValue) < percentageValue
 
 /// <summary>
-/// Opens a stream for a specified sample resource file. 
+/// Opens a stream for a specified sample resource file.
 /// </summary>
 let sampleFileStream sampleFileName =
     let assembly = Assembly.GetExecutingAssembly()
-    assembly.GetManifestResourceStream
-        ("Demeton.Tests.samples." + sampleFileName)
+
+    assembly.GetManifestResourceStream(
+        "Demeton.Tests.samples." + sampleFileName
+    )
diff --git a/Demeton.Tests/Water bodies/Load water bodies data.fs b/Demeton.Tests/Water bodies/Load water bodies data.fs
index f7c1b3d..e520dc2 100644
--- a/Demeton.Tests/Water bodies/Load water bodies data.fs	
+++ b/Demeton.Tests/Water bodies/Load water bodies data.fs	
@@ -20,13 +20,15 @@ open Swensen.Unquote
 open Demeton.WorldCover.Types
 open Demeton.Dem.Funcs
 open FileSys
-
+open Demeton.WorldCover.Fetch
 
 type WaterBodiesTile = HeightsArray
 
 [<Literal>]
 let WaterBodiesTileSize = WorldCoverTileSize
 
+[<Literal>]
+let WaterBodiesCacheSubdirName = "WaterBodies"
 
 
 /// <summary>
@@ -64,10 +66,13 @@ let encodeWaterBodiesHeightsArrayToPng
     outputStream |> Png.File.savePngToStream ihdr imageData
 
 
-let decodeWaterBodiesTileFromPng tileId stream =
+let decodeWaterBodiesTileFromPng tileSize tileId stream =
     let validateImageSize (ihdr: IhdrData) =
-        match (ihdr.Width, ihdr.Height) with
-        | WaterBodiesTileSize, WaterBodiesTileSize -> Ok ihdr
+        match ihdr.Width, ihdr.Height with
+        | tileWidth, tileHeight when
+            tileWidth = tileSize && tileHeight = tileSize
+            ->
+            Ok ihdr
         | _, _ ->
             Error
                 "The image size of this PNG does not correspond to the water bodies tile."
@@ -87,7 +92,7 @@ let decodeWaterBodiesTileFromPng tileId stream =
                 "The bit depth of this PNG does not correspond to the water bodies tile."
 
     let generateHeightsArray (ihdr: IhdrData) (imageData: RawImageData) =
-        let minX, minY = tileMinCell WaterBodiesTileSize tileId
+        let minX, minY = tileMinCell tileSize tileId
 
         let waterBodiesTileInitialize (cells: DemHeight[]) =
             let totalPixelsCount = ihdr.Width * ihdr.Height
@@ -115,8 +120,8 @@ let decodeWaterBodiesTileFromPng tileId stream =
             HeightsArray(
                 minX,
                 minY,
-                WaterBodiesTileSize,
-                WaterBodiesTileSize,
+                tileSize,
+                tileSize,
                 HeightsArrayCustomInitializer waterBodiesTileInitialize
             )
         )
@@ -137,46 +142,79 @@ let decodeWaterBodiesTileFromPng tileId stream =
 
 let loadWaterBodiesTile
     (fileExists: FileExistsChecker)
+    (ensureDirectoryExists: DirectoryExistsEnsurer)
+    (openFileToRead: FileReader)
+    (openFileToWrite: FileWriter)
+    decodeWaterBodiesTileFromPng
+    downloadFile
     cacheDir
+    (availableWorldCoverTiles: Set<DemTileId>)
     (tileId: DemTileId)
     : WaterBodiesTile option =
-    // todo 10: implement this function
-    let cachedFileName =
-        Path.Combine(cacheDir, $"WaterBodies-%s{toTileName tileId}.png")
-
-    if fileExists cachedFileName then
-        // todo 5: implement reading of water bodies PNG tile
-        HeightsArray(
-            0,
-            0,
-            WaterBodiesTileSize,
-            WaterBodiesTileSize,
-            EmptyHeightsArray
+    let cachedPngFileName =
+        Path.Combine(
+            cacheDir,
+            WaterBodiesCacheSubdirName,
+            $"WaterBodies-%s{toTileName tileId}.png"
         )
-        |> Some
+
+    let cachedNoneFileName = Path.ChangeExtension(cachedPngFileName, "none")
+
+    if fileExists cachedPngFileName then
+        match openFileToRead cachedPngFileName with
+        | Ok stream ->
+            decodeWaterBodiesTileFromPng WaterBodiesTileSize tileId stream
+            |> Some
+        | Error error -> Error error |> raise error.Exception
+    else if fileExists cachedNoneFileName then
+        None
     else
-        NotImplementedException() |> raise
+        // todo 10: construct PNG from WorldCover tile
+        let containingWorldCoverTileId = containingWorldCoverFileTileId tileId
+
+        if
+            (availableWorldCoverTiles |> Set.contains containingWorldCoverTileId)
+        then
+            Should.reportNotImplemented "Load world cover tile"
+        else
+            match ensureDirectoryExists cacheDir with
+            | Ok _ -> ()
+            | Error error -> Error error |> raise error.Exception
+
+            // Create a 'none' file and return None
+            match openFileToWrite cachedNoneFileName with
+            | Ok stream ->
+                stream |> closeStream
+                None
+            | Error error -> Error error |> raise error.Exception
 
 
 
 [<Literal>]
-let cacheDir = "cache"
+let CacheDir = "cache"
+
+let someWorldCoverTiles = set []
 
+/// <summary>
+/// Tests that the water bodies heights array can be encoded into PNG via
+/// a stream and then decoded back to the heights array.
+/// </summary>
 [<Fact>]
 let ``Water bodies heights array can be encoded into PNG and back`` () =
+    let tileSize = 100
     let tileId = demTileXYId 7 45
-    let minX = longitudeToCellX WaterBodiesTileSize 7
-    let minY = latitudeToCellY WaterBodiesTileSize 45
+    let minX = longitudeToCellX tileSize 7
+    let minY = latitudeToCellY tileSize 45
 
-    let rnd = Random(Seed=123)
+    let rnd = Random(Seed = 123)
 
     let waterBodiesData =
         HeightsArray(
             minX,
             minY,
-            WaterBodiesTileSize,
-            WaterBodiesTileSize,
-            HeightsArrayInitializer1D (fun _ -> rnd.Next(0, 2) |> int16)
+            tileSize,
+            tileSize,
+            HeightsArrayInitializer1D(fun _ -> rnd.Next(0, 2) |> int16)
         )
 
     let stream =
@@ -184,7 +222,7 @@ let ``Water bodies heights array can be encoded into PNG and back`` () =
 
     stream.Seek(0L, SeekOrigin.Begin) |> ignore
 
-    let decodingResult = decodeWaterBodiesTileFromPng tileId stream
+    let decodingResult = decodeWaterBodiesTileFromPng tileSize tileId stream
 
     match decodingResult with
     | Ok decodedWaterBodiesData ->
@@ -192,17 +230,14 @@ let ``Water bodies heights array can be encoded into PNG and back`` () =
         test <@ decodedWaterBodiesData.MinY = waterBodiesData.MinY @>
         test <@ decodedWaterBodiesData.Width = waterBodiesData.Width @>
         test <@ decodedWaterBodiesData.Height = waterBodiesData.Height @>
-        test <@ decodedWaterBodiesData.Cells.Length = waterBodiesData.Cells.Length @>
-
-        // sample random cells and compare them
-        Array.zeroCreate 1000
-        |> Array.map (fun _ -> rnd.Next(0, decodedWaterBodiesData.Cells.Length))
-        |> Array.iter (fun i ->
-            test <@ decodedWaterBodiesData.Cells[i] = waterBodiesData.Cells[i] @>)
-    | Error error -> failwith error
-
 
+        test
+            <@
+                decodedWaterBodiesData.Cells.Length = waterBodiesData.Cells.Length
+            @>
 
+        test <@ decodedWaterBodiesData.Cells = waterBodiesData.Cells @>
+    | Error error -> failwith error
 
 
 // todo 30: implement this test
@@ -217,10 +252,81 @@ let ``Returns None if the tile is not covered by the underlying WorldCover tiles
 [<Fact>]
 let ``If water bodies PNG tile is already in cache, return that one`` () =
     let fileExists fileName =
-        test <@ fileName = @"cache\WaterBodies-N45E007.png" @>
+        test <@ fileName = @"cache\WaterBodies\WaterBodies-N45E007.png" @>
         true
 
+    let openFileToRead fileName =
+        test <@ fileName = @"cache\WaterBodies\WaterBodies-N45E007.png" @>
+        Ok(new MemoryStream() :> Stream)
+
+    let decodeWaterBodiesTileFromPng tileSize tileId stream =
+        test <@ tileSize = WaterBodiesTileSize @>
+        HeightsArray(0, 0, 1, 1, HeightsArrayInitializer1D(fun _ -> 0s))
+
     let waterBodiesTile =
-        loadWaterBodiesTile fileExists cacheDir (demTileXYId 7 45)
+        loadWaterBodiesTile
+            fileExists
+            Should.notBeCalled
+            openFileToRead
+            Should.notBeCalled
+            decodeWaterBodiesTileFromPng
+            Should.notBeCalled2
+            CacheDir
+            someWorldCoverTiles
+            (demTileXYId 7 45)
 
     test <@ waterBodiesTile.IsSome @>
+
+[<Fact>]
+let ``If water bodies 'none' file is in cache, return None for the tile`` () =
+    let fileExists fileName =
+        match fileName with
+        | @"cache\WaterBodies\WaterBodies-N45E007.none" -> true
+        | _ -> false
+
+    let waterBodiesTile =
+        loadWaterBodiesTile
+            fileExists
+            Should.notBeCalled
+            Should.notBeCalled
+            Should.notBeCalled
+            Should.notBeCalled2
+            Should.notBeCalled2
+            CacheDir
+            someWorldCoverTiles
+            (demTileXYId 7 45)
+
+    test <@ waterBodiesTile.IsNone @>
+
+[<Fact>]
+let ``If 1x1 tile has no corresponding WorldCover tile, make 'none' file and return None``
+    ()
+    =
+    let fileExists fileName = false
+
+    let ensureDirExists dirName = Ok dirName
+
+    let openFileToRead fileName =
+        raise <| AssertionFailedException("Should not be called.")
+
+    let openFileToWrite fileName =
+        test <@ fileName = @"cache\WaterBodies\WaterBodies-N45E007.none" @>
+
+        new MemoryStream() :> Stream |> Ok
+
+    let downloadFile (url: string) fileName =
+        Should.fail "Unexpected file download"
+
+    let waterBodiesTile =
+        loadWaterBodiesTile
+            fileExists
+            ensureDirExists
+            openFileToRead
+            openFileToWrite
+            Should.notBeCalled2
+            downloadFile
+            CacheDir
+            someWorldCoverTiles
+            (demTileXYId 7 45)
+
+    test <@ waterBodiesTile.IsNone @>
diff --git a/Demeton.sln b/Demeton.sln
index 3827343..93f0763 100644
--- a/Demeton.sln
+++ b/Demeton.sln
@@ -26,6 +26,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 		build.ps1 = build.ps1
 		.config\dotnet-tools.json = .config\dotnet-tools.json
 		zip-dropbox.ps1 = zip-dropbox.ps1
+		.editorconfig = .editorconfig
 	EndProjectSection
 EndProject
 Project("{6EC3EE1D-3C4E-46DD-8F32-0CC8E7565705}") = "Demeton", "Demeton\Demeton.fsproj", "{74E691F7-30D6-4D0B-AF9A-D9738C901E3A}"
diff --git a/Demeton/FileSys.fs b/Demeton/FileSys.fs
index 143a1ee..b0a3d29 100644
--- a/Demeton/FileSys.fs
+++ b/Demeton/FileSys.fs
@@ -1,5 +1,5 @@
 /// <summary>
-/// kContains functions for working with files and directories
+/// Contains functions for working with files and directories
 /// in a functional way.
 /// </summary>
 module FileSys
diff --git a/Demeton/WorldCover/Fetch.fs b/Demeton/WorldCover/Fetch.fs
index ffba359..60f8415 100644
--- a/Demeton/WorldCover/Fetch.fs
+++ b/Demeton/WorldCover/Fetch.fs
@@ -41,7 +41,7 @@ let ensureGeoJsonFile
 /// geoJSON file.
 /// </summary>
 let listAllAvailableFiles
-    (openFileToRead: FileWriter)
+    (openFileToRead: FileReader)
     (geoJsonFile: string)
     : DemTileId seq =
     match openFileToRead geoJsonFile with