diff --git a/2021/ruby/day_15.rb b/2021/ruby/day_15.rb
new file mode 100644
index 0000000..e22acb0
--- /dev/null
+++ b/2021/ruby/day_15.rb
@@ -0,0 +1,38 @@
+cavern = ARGF.read.split("\n").map { _1.chars.map(&:to_i) }
+cavern = (0..4).flat_map {|i|
+  cavern.map {|row| row.map { (_1 + i - 1) % 9 + 1 }}
+}
+cavern = cavern.map {|row|
+  (0..4).flat_map {|i|
+    row.map { (_1 + i - 1) % 9 + 1 }
+  }
+}
+
+cavern = cavern
+  .flat_map.with_index {|row,y| row.map.with_index {|risk,x| [[y,x], risk] }}
+  .to_h
+bottom_right = cavern.keys.max
+
+risks = Hash.new(0)
+risks[[0,0]] = 0
+edges = [[0,0]]
+
+until risks.has_key?(bottom_right)
+  yx = edges.min_by { risks.fetch(_1) }
+  edges.delete(yx)
+
+  y,x = yx
+  neighbors = [[-1, 0], [1, 0], [0, 1], [0, -1]]
+    .map {|dy,dx| [y+dy, x+dx] }
+    .select { cavern.has_key?(_1) }
+    .reject { risks.has_key?(_1) }
+
+  risk = risks.fetch(yx)
+  neighbors.each do
+    risks[_1] = risk + cavern.fetch(_1)
+  end
+
+  edges.concat(neighbors)
+end
+
+p risks[bottom_right]