Dungeon Generation in the Gnomish Mines of NetHack

# Dungeon Generation in the Gnomish Mines of NetHack Source: [https://www.boristhebrave.com/2026/10/18/dungeon-generation-in-the-gnomish-mines-of-nethack/](https://www.boristhebrave.com/2026/10/18/dungeon-generation-in-the-gnomish-mines-of-nethack/) [Skip to content](https://www.boristhebrave.com/2026/10/18/dungeon-generation-in-the-gnomish-mines-of-nethack/#content) Ok, this one is really best explained with a visualiser: NetHack’s level generation has a lot of interesting things, but I wanted to focus on the[Gnomish Mines](https://nethackwiki.com/wiki/Gnomish_Mines)because of their use of Cellular Automata\. This gives it an organic caves feel, while the majority of the game is spent in square rooms\. It’s very straightforward: - Randomly pick 40% of the cells to be floor - Apply four passes of Cellular Automata - Fix up disconnected caves with extra corridors Let’s dive into the cellular automata specifically\. There’s only two states – floor or rock\. And each pass works by counting how many floor tiles are in the 8\-neighbor hood of each cell\. Pass 1: 0\-2 → rock; 3\-4 → unchanged, 5\-8 → floor Pass 2: 0\-4 → unchanged; 5 → rock; 6\-8 → unchanged Pass 3: 0\-2 → rock; 3\-8 → unchanged\. Pass 4:*same as pass 3* Interestingly, pass 1 is unbuffered\. Each cell is updated in place before moving to the next cell\. All the rest are buffered – all cells have new computed values, then the entire grid updated at once\. Pass 1 creates floors near other floors, and creates rocks near other rocks\. I’d call this a “clustering” pass, that pushes the random tiles towards wide open areas of tiles\. The unbuffering is important here so that you don’t flop\-flop, turning something rock→floor near something turning floor→rock\. Passes 3 and 4 are erosion routines, it’s a very common use of cellular automata\. If you have 6\-8 rocks nearby, then you also become a rock\. That tends to smooth out corners, and delete tiny islands\. Pass 2 though… It turns cells to rock if they have exactly 5 floor neighbors \(i\.e\. 3 rock neighbors\)\. I’m still struggling to understand what purpose this is intended to achieve\. Any ideas?