diff --git a/2021/ruby/day_09.rb b/2021/ruby/day_09.rb
new file mode 100644
index 0000000..e90b2bf
--- /dev/null
+++ b/2021/ruby/day_09.rb
@@ -0,0 +1,31 @@
+require "set"
+
+INPUT = ARGF.read.split("\n").map {|l| l.chars.map(&:to_i) }
+
+NEIGHBORS = [[0,-1], [-1, 0], [1, 0], [0, 1]]
+def neighbors(y,x)
+  NEIGHBORS
+    .map {|dy,dx| [y+dy, x+dx] }
+    .select {|y,x| (0...INPUT.size).cover?(y) && (0...INPUT[0].size).cover?(x) }
+end
+
+all = (0...INPUT.size).flat_map {|y| (0...INPUT[0].size).map {|x| [y, x]}}
+lows = all.select {|y,x| neighbors(y,x).all? {|yy,xx| INPUT[yy][xx] > INPUT[y][x] }}
+# p lows.sum {|y,x| INPUT[y][x] + 1 }
+
+def fill_basin(low)
+  basin = Set.new([low])
+
+  stack = [low]
+  until stack.empty?
+    y,x = stack.shift
+    new_neighbors = neighbors(y,x).reject {|y,x| basin.include?([y,x]) || INPUT[y][x] == 9 }
+    stack.concat(new_neighbors)
+    basin.merge(new_neighbors)
+  end
+
+  basin
+end
+
+p lows.map { fill_basin(_1) }.map(&:size).sort[-3, 3].inject(:*)
+