diff --git a/src/main/scala/de.qwertyuiop.aoc/2021/day09.scala b/src/main/scala/de.qwertyuiop.aoc/2021/day09.scala
index 710c992..78b91e0 100644
--- a/src/main/scala/de.qwertyuiop.aoc/2021/day09.scala
+++ b/src/main/scala/de.qwertyuiop.aoc/2021/day09.scala
@@ -1,6 +1,47 @@
 package de.qwertyuiop.aoc.`2021`
 
 import de.qwertyuiop.aoc.lib.*
+
 import cats.*, cats.implicits.given
 
-def day9(using InputSource): Unit = ???
+def day9(using InputSource): Unit =
+  val heights = HeightMap(input(_.map(_ - '0').toVector).toVector)
+
+  val lowPoints =
+    for
+      x <- 0 until heights.sizeX
+      y <- 0 until heights.sizeY
+      c = heights(x, y)
+      if heights.neighbourValues(x, y).forall(_ > c)
+    yield ((x, y), c)
+
+  val riskLevel = lowPoints.map(_(1)).sum + lowPoints.size
+  println(s"Risk level: $riskLevel")
+
+  def findBasin(heights: HeightMap, start: (Int, Int)): Set[(Int, Int)] =
+    val upwards = heights.neighbourCoords(start).filter(c => { val h = heights(c); h > heights(start) && h < 9 })
+    (upwards.toSet) + start ++ upwards.flatMap(n => findBasin(heights, n))
+
+  val largestBasinSizes = lowPoints.map((low, _) => findBasin(heights, low).size).sorted(summon[Ordering[Int]].reverse).take(3)
+  println(s"Product of largest basins: ${largestBasinSizes.product}")
+
+
+case class HeightMap(heights: Vector[Vector[Int]]):
+  val sizeX = heights.size
+  val sizeY = heights(0).size
+  export heights.apply
+
+  def apply(pos: (Int, Int)): Int = heights(pos(0))(pos(1))
+
+  def neighbourValues(x: Int, y: Int): Vector[Int] =
+    neighbourCoords((x, y)).map(apply)
+
+  /* Can't reuse implementation from last year in Vec2D, as it also considers diagonals */
+  def neighbourCoords(pos: (Int, Int)): Vector[(Int, Int)] =
+    val (x,y) = pos
+    val vb = collection.immutable.VectorBuilder[(Int, Int)]()
+    if x > 0 then vb += ((x - 1, y))
+    if x < sizeX - 1 then vb += ((x + 1, y))
+    if y > 0 then vb += ((x, y - 1))
+    if y < sizeY - 1 then vb += ((x, y + 1))
+    vb.result()