The Subterranean Ocean is a watery branch of the main dungeon. It is accessible from a down-stair at dungeon level 2, 3, or 4 (not from the same level as the entrance to the Gnomish Mines) and goes downward from there, rejoining the Mazes of Menace at a much lower depth. Monster generation in this branch is biased toward chaotic monsters. The Ocean consists of seven or eight levels, including two special levels (randomly selected from six possibilities). The bottom level of the Seas contains a stair down to a much lower level of the Mazes of Menace - specifically, to a second up stair located somewhere between level 20 and the last level above Medusa's Island (which occurs between level 21 and 28). The up stair is not on the same level as either the quest portal or the portal to Fort Ludios.
This means that passing through the Ocean takes just 7-8 levels to get anywhere from 16 to 25 levels below your starting point. This could be helpful on a dig for victory, but be warned that for the unprepared adventurer, the Ocean can be significantly more difficult to descend - and more dangerous to explore - than the regular dungeon.
The exact layout of the Ocean is:
- three normal, randomly-generated Ocean levels
- a randomly selected special Ocean level from among the six listed below
- one or two more normal Ocean levels
- a second (different) randomly selected special level
- one more normal level containing the down stairs to return you to the main branch.
All levels are eligible to leave bones. The walls and floors are diggable (except where specified on a couple of the special levels), but digging down anywhere in the branch has a 20% chance of creating a moat square, even if not adjacent to existing water.
A "normal" level in the Subterranean Ocean consists of either randomly drawn irregular caves (similar to the Gnomish Mines) with large pools and rivers of water, or else an entire screen of water with several randomly drawn irregular islands. In the later case, the upstairs and the downstairs will always be on different islands, and you will need some way of crossing the water to reach the stairs to the next level. As in the Mines, for each level there is an independent 50% chance that it will be unlit. Each level contains two piranhas, two giant eels, two jellyfish, and one random ;.
Selected from among these possibilities:
- Paradise Island - A tropical desert isle, complete with volcano.
- The Sunless Sea - Modified version of the SLASH'EM level.
- The Caves of Ice - Complete with a "damsel with a dulcimer."
- Lost Atlantis - The remains of an ancient civilization, guarded by mermen and mermaids.
- Yerebatan Sarayi - Based on the sunken palace beneath Istanbul (formerly Constantinople).
- Cascade Falls - A thundering waterfall.