diff --git a/2021/README.md b/2021/README.md
index 440a86a..ffac725 100644
--- a/2021/README.md
+++ b/2021/README.md
@@ -15,7 +15,7 @@
 |  S  |  M  |  T  |  W  |  T  |  F  |  S  |
 | :-: | :-: | :-: | :-: | :-: | :-: | :-: |
 |     |     |     | [1] | [2] | [3] | [4] |
-| [5] | [6] | [7] | [8] |  9  | 10  | 11  |
+| [5] | [6] | [7] | [8] | [9] | 10  | 11  |
 | 12  | 13  | 14  | 15  | 16  | 17  | 18  |
 | 19  | 20  | 21  | 22  | 23  | 24  | 25  |
 
@@ -29,3 +29,4 @@
 [6]: ./lib/2021/6.ex
 [7]: ./lib/2021/7.ex
 [8]: ./lib/2021/8.ex
+[9]: ./lib/2021/9.ex
diff --git a/2021/lib/2021/9.ex b/2021/lib/2021/9.ex
new file mode 100644
index 0000000..bed7b87
--- /dev/null
+++ b/2021/lib/2021/9.ex
@@ -0,0 +1,89 @@
+import AOC
+
+aoc 2021, 9 do
+  def input_map(),
+    do:
+      input_string()
+      |> String.split("\n")
+      |> Enum.with_index()
+      |> Enum.flat_map(fn {line, i} ->
+        line
+        |> String.split("", trim: true)
+        |> Enum.map(&String.to_integer/1)
+        |> Enum.with_index()
+        |> Enum.map(fn {cell, j} -> {{i, j}, cell} end)
+      end)
+      |> Map.new()
+
+  def n({i, j}), do: {i - 1, j}
+  def e({i, j}), do: {i, j + 1}
+  def s({i, j}), do: {i + 1, j}
+  def w({i, j}), do: {i, j - 1}
+
+  def at(map, pos), do: Map.get(map, pos, :infinity)
+
+  def directions(), do: [&n/1, &e/1, &s/1, &w/1]
+
+  def neighbors(map, origin) do
+    for d <- directions(), pos = d.(origin) do
+      {pos, at(map, pos)}
+    end
+  end
+
+  def gt({_, a}, b), do: a > b
+
+  def basin_from(map, pos, seen \\ MapSet.new()) do
+    curr = at(map, pos)
+
+    if min(curr, 9) == 9 do
+      MapSet.new()
+    else
+      seen = MapSet.put(seen, pos)
+
+      inclines =
+        neighbors(map, pos)
+        |> Enum.filter(&gt(&1, curr))
+        |> Enum.map(&elem(&1, 0))
+
+      for inc <- inclines, reduce: seen do
+        seen ->
+          if MapSet.member?(seen, inc) do
+            seen
+          else
+            MapSet.union(seen, basin_from(map, inc, seen))
+          end
+      end
+    end
+  end
+
+  def p1 do
+    map = input_map()
+
+    for i <- 0..99, j <- 0..99 do
+      pos = {i, j}
+      curr = at(map, pos)
+
+      neighbors =
+        directions()
+        |> Enum.map(fn d -> at(map, d.(pos)) end)
+
+      if Enum.all?(neighbors, &(curr < &1)) do
+        curr + 1
+      else
+        0
+      end
+    end
+    |> Enum.sum()
+  end
+
+  def p2 do
+    map = input_map()
+
+    for i <- 0..99, j <- 0..99, pos = {i, j} do
+      basin_from(map, pos) |> MapSet.size()
+    end
+    |> Enum.sort(:desc)
+    |> Enum.take(3)
+    |> Enum.product()
+  end
+end