diff --git a/2022/src/day18.rs b/2022/src/day18.rs
index ee76db2..6ddc473 100644
--- a/2022/src/day18.rs
+++ b/2022/src/day18.rs
@@ -1,4 +1,6 @@
+use ahash::AHashMap;
 use ahash::AHashSet;
+use anyhow::Context;
 use anyhow::Result;
 use nom::bytes::complete::tag;
 use nom::character::complete::newline;
@@ -51,8 +53,81 @@ pub fn part1(input: &[u8]) -> Result<String> {
     Ok(faces.to_string())
 }
 
-pub fn part2(_input: &[u8]) -> Result<String> {
-    anyhow::bail!("not implemented")
+pub fn part2(input: &[u8]) -> Result<String> {
+    let voxels = parse_input(input, parse_voxels)?;
+    let max = voxels
+        .iter()
+        .copied()
+        .flatten()
+        .max()
+        .context("No voxels?")?;
+
+    let mut outside = AHashMap::new();
+    let mut outside_candidates = AHashSet::new();
+    let mut todo = Vec::new();
+
+    let mut is_outside = |voxel: [u8; 3]| {
+        if let Some(&state) = outside.get(&voxel) {
+            return state;
+        }
+
+        let mut is_outside = false;
+
+        todo.push(voxel);
+        outside_candidates.insert(voxel);
+
+        'outer: while let Some(voxel) = todo.pop() {
+            for axis in 0..3 {
+                for offset in [-1, 1] {
+                    let mut to_search = voxel;
+                    if let Some(new_coord) = to_search[axis].checked_add_signed(offset) {
+                        to_search[axis] = new_coord;
+
+                        if voxels.contains(&to_search) {
+                            // Filled voxels cannot lead outside
+                            continue;
+                        } else if new_coord >= max {
+                            is_outside = true;
+                            break 'outer;
+                        } else if let Some(&state) = outside.get(&to_search) {
+                            is_outside = state;
+                            break 'outer;
+                        } else if outside_candidates.insert(to_search) {
+                            todo.push(to_search);
+                        }
+                    } else {
+                        // Managed to get below zero, which is outside
+                        is_outside = true;
+                        break 'outer;
+                    }
+                }
+            }
+        }
+
+        let map = |voxel| (voxel, is_outside);
+
+        outside.extend(todo.drain(..).map(map));
+        outside.extend(outside_candidates.drain().map(map));
+
+        is_outside
+    };
+
+    let mut faces = 0;
+
+    for &voxel in &voxels {
+        for axis in 0..3 {
+            for offset in [-1, 1] {
+                let mut to_search = voxel;
+                to_search[axis] = to_search[axis].wrapping_add_signed(offset);
+
+                if !voxels.contains(&to_search) && is_outside(to_search) {
+                    faces += 1;
+                }
+            }
+        }
+    }
+
+    Ok(faces.to_string())
 }
 
 #[cfg(test)]
@@ -65,4 +140,9 @@ mod tests {
     fn sample_part1() {
         assert_eq!(part1(SAMPLE).unwrap(), "64");
     }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!(part2(SAMPLE).unwrap(), "58");
+    }
 }