diff --git a/src/AdventOfCode/Day22.cs b/src/AdventOfCode/Day22.cs
new file mode 100644
index 0000000..7043b4c
--- /dev/null
+++ b/src/AdventOfCode/Day22.cs
@@ -0,0 +1,205 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using AdventOfCode.Utilities;
+
+namespace AdventOfCode
+{
+    /// <summary>
+    /// Solver for Day 22
+    /// </summary>
+    public class Day22
+    {
+        public int Part1(string[] input)
+        {
+            var cubes = input.Select((line, i) => SandCube.Parse(i, line)).ToArray();
+            var tower = SandCubeTower.Build(cubes);
+
+            int total = 0;
+
+            foreach (SandCube cube in cubes)
+            {
+                if (!tower.Supporting.TryGetValue(cube.Id, out ISet<int> holdingUp))
+                {
+                    // cube doesn't support anything, safe to remove
+                    total++;
+                    continue;
+                }
+
+                if (holdingUp.All(h => tower.SupportedBy[h].Count > 1))
+                {
+                    // we're supporting things, but everything we're supporting is itself supported by at least one other thing
+                    total++;
+                }
+            }
+
+            return total;
+        }
+
+        public int Part2(string[] input)
+        {
+            var cubes = input.Select((line, i) => SandCube.Parse(i, line)).ToArray();
+            var tower = SandCubeTower.Build(cubes);
+
+            int total = 0;
+
+            foreach (SandCube cube in cubes)
+            {
+                Queue<int> queue = new();
+                HashSet<int> fell = new() { cube.Id };
+
+                queue.Enqueue(cube.Id);
+
+                while (queue.Count > 0)
+                {
+                    int id = queue.Dequeue();
+
+                    if (!tower.Supporting.TryGetValue(id, out ISet<int> holdingUp))
+                    {
+                        // not holding anything up, so nothing to fall
+                        continue;
+                    }
+
+                    foreach (int heldUp in holdingUp)
+                    {
+                        if (tower.SupportedBy[heldUp].All(fell.Contains))
+                        {
+                            // everything supporting this one fell down
+                            queue.Enqueue(heldUp);
+                            fell.Add(heldUp);
+                        }
+                    }
+                }
+
+                total += fell.Count - 1; // don't count the block that we removed
+            }
+
+            return total;
+        }
+
+        /// <summary>
+        /// Tower of sand cubes
+        /// </summary>
+        /// <param name="Supporting">Lookup of each sand cube to which other sand cubes it is supporting</param>
+        /// <param name="SupportedBy">Lookup of each sand cube to which other sand cubes it is supported by</param>
+        private record SandCubeTower(IDictionary<int, ISet<int>> Supporting, IDictionary<int, ISet<int>> SupportedBy)
+        {
+            /// <summary>
+            /// Building the tower from the given starting cube positions representing falling cubes
+            /// </summary>
+            /// <param name="falling">Falling cubes</param>
+            /// <returns>Sand cube tower after all cubes have fallen and settled</returns>
+            public static SandCubeTower Build(IEnumerable<SandCube> falling)
+            {
+                Dictionary<int, ISet<int>> supporting = new();
+                Dictionary<int, ISet<int>> supportedBy = new();
+                Dictionary<Point3D, int> occupiedSpace = new();
+
+                foreach (SandCube cube in falling.OrderBy(c => c.BottomLeftFront.Z))
+                {
+                    SandCube current = cube;
+                    SandCube dropped = cube.Drop();
+
+                    // drop until we hit either the ground or a point occupied by another cube
+                    while (dropped.BottomLayer().All(p => p.Z > 0 && !occupiedSpace.ContainsKey(p)))
+                    {
+                        current = dropped;
+                        dropped = current.Drop();
+                    }
+
+                    // mark the cubes below as supporting this one
+                    foreach (Point3D point in dropped.BottomLayer().Where(occupiedSpace.ContainsKey))
+                    {
+                        int supportingId = occupiedSpace[point];
+                        supporting.GetOrCreate(supportingId, () => new HashSet<int>()).Add(current.Id);
+                        supportedBy.GetOrCreate(current.Id, () => new HashSet<int>()).Add(supportingId);
+                    }
+
+                    // settle this cube in place
+                    foreach (Point3D point in current.Points())
+                    {
+                        occupiedSpace[point] = current.Id;
+                    }
+                }
+
+                return new SandCubeTower(supporting, supportedBy);
+            }
+        }
+
+        /// <summary>
+        /// A cube of sand
+        /// </summary>
+        /// <param name="Id">Cube ID</param>
+        /// <param name="BottomLeftFront">The bottom front left corner</param>
+        /// <param name="TopRightBack">The top right back corner</param>
+        private record SandCube(int Id, Point3D BottomLeftFront, Point3D TopRightBack)
+        {
+            /// <summary>
+            /// Parse a sand cube from input
+            /// </summary>
+            /// <param name="id">Cube ID</param>
+            /// <param name="input">Input line</param>
+            /// <returns>Sand cube</returns>
+            public static SandCube Parse(int id, string input)
+            {
+                int[] numbers = input.Numbers<int>();
+
+                Point3D bottomLeftFront = (Math.Min(numbers[0], numbers[3]),
+                                           Math.Min(numbers[1], numbers[4]),
+                                           Math.Min(numbers[2], numbers[5]));
+
+                Point3D topRightBack = (Math.Max(numbers[0], numbers[3]),
+                                        Math.Max(numbers[1], numbers[4]),
+                                        Math.Max(numbers[2], numbers[5]));
+
+                return new SandCube(id, bottomLeftFront, topRightBack);
+            }
+
+            /// <summary>
+            /// Drop the current cube by one place
+            /// </summary>
+            /// <returns>Same cube but space one lower down</returns>
+            public SandCube Drop()
+            {
+                return this with
+                {
+                    BottomLeftFront = this.BottomLeftFront - (0, 0, 1),
+                    TopRightBack = this.TopRightBack - (0, 0, 1)
+                };
+            }
+
+            /// <summary>
+            /// Enumerate all the points of space taken up by this cube
+            /// </summary>
+            /// <returns>Cube points</returns>
+            public IEnumerable<Point3D> Points()
+            {
+                for (int x = this.BottomLeftFront.X; x <= this.TopRightBack.X; x++)
+                {
+                    for (int y = this.BottomLeftFront.Y; y <= this.TopRightBack.Y; y++)
+                    {
+                        for (int z = this.BottomLeftFront.Z; z <= this.TopRightBack.Z; z++)
+                        {
+                            yield return (x, y, z);
+                        }
+                    }
+                }
+            }
+
+            /// <summary>
+            /// Enumerate all the points of space taken up by the bottom layer of this cube
+            /// </summary>
+            /// <returns>Bottom layer</returns>
+            public IEnumerable<Point3D> BottomLayer()
+            {
+                for (int x = this.BottomLeftFront.X; x <= this.TopRightBack.X; x++)
+                {
+                    for (int y = this.BottomLeftFront.Y; y <= this.TopRightBack.Y; y++)
+                    {
+                        yield return (x, y, this.BottomLeftFront.Z);
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/src/AdventOfCode/inputs/day22.txt b/src/AdventOfCode/inputs/day22.txt
new file mode 100644
index 0000000..c38b88b
Binary files /dev/null and b/src/AdventOfCode/inputs/day22.txt differ
diff --git a/tests/AdventOfCode.Tests/Day22Tests.cs b/tests/AdventOfCode.Tests/Day22Tests.cs
new file mode 100644
index 0000000..45fa637
--- /dev/null
+++ b/tests/AdventOfCode.Tests/Day22Tests.cs
@@ -0,0 +1,81 @@
+using System.IO;
+using Xunit;
+using Xunit.Abstractions;
+
+
+namespace AdventOfCode.Tests
+{
+    public class Day22Tests
+    {
+        private readonly ITestOutputHelper output;
+        private readonly Day22 solver;
+
+        public Day22Tests(ITestOutputHelper output)
+        {
+            this.output = output;
+            this.solver = new Day22();
+        }
+
+        private static string[] GetRealInput()
+        {
+            string[] input = File.ReadAllLines("inputs/day22.txt");
+            return input;
+        }
+
+        private static string[] GetSampleInput()
+        {
+            return new string[]
+            {
+                "1,0,1~1,2,1",
+                "0,0,2~2,0,2",
+                "0,2,3~2,2,3",
+                "0,0,4~0,2,4",
+                "2,0,5~2,2,5",
+                "0,1,6~2,1,6",
+                "1,1,8~1,1,9",
+            };
+        }
+
+        [Fact]
+        public void Part1_SampleInput_ProducesCorrectResponse()
+        {
+            var expected = 5;
+
+            var result = solver.Part1(GetSampleInput());
+
+            Assert.Equal(expected, result);
+        }
+
+        [Fact]
+        public void Part1_RealInput_ProducesCorrectResponse()
+        {
+            var expected = 395;
+
+            var result = solver.Part1(GetRealInput());
+            output.WriteLine($"Day 22 - Part 1 - {result}");
+
+            Assert.Equal(expected, result);
+        }
+
+        [Fact]
+        public void Part2_SampleInput_ProducesCorrectResponse()
+        {
+            var expected = 7;
+
+            var result = solver.Part2(GetSampleInput());
+
+            Assert.Equal(expected, result);
+        }
+
+        [Fact]
+        public void Part2_RealInput_ProducesCorrectResponse()
+        {
+            var expected = 64714;
+
+            var result = solver.Part2(GetRealInput());
+            output.WriteLine($"Day 22 - Part 2 - {result}");
+
+            Assert.Equal(expected, result);
+        }
+    }
+}