Beat Dungeon Slumps With Dungeon Sumps

Roleplaying Tips Newsletter #1099

The first issue of 5 Room Dungeons Zine features underwater gameplay in a location you would not expect to have to swim or fight.

Part of the reason I made this unusual environment the 5RD’s Feature Location is to challenge your players to master the underwater rules.

When your group learns edge case type rules like this, you can really liven up your game with them ever after.

In this case, one obvious approach would be seaborne adventures. Pirates, perhaps. Navy. Or coastal exploration like in my Barbossa campaign.

We can also fill our dungeons with water.

Today I want to share with you an idea for your games that offers an under-used feature of dungeons:

The Sump

Imagine walking along a tunnel, through a cave, or down a corridor and the floor suddenly drops.

The ground might have slumped. Or geological forces have shifted the area. Or the architects were following softer rock.

And in this slumped or depressed area water has gathered.

That’s a sump.

You might know a sump as something under your sink. That curvy pipe helps with pressure regulation. Here’s a visual based on a 2018 cave rescue, so you can see what we’re talking about:

Those tunnel dips filled with water are sumps.

Well, let’s rub our GMing hands together and cackle with glee, because we can get a lot of mileage out of sumps in our adventures, pun mightily intended!

Why Sumps?

The main goals for using sumps are:

  • Find more ways to stage underwater encounters to cement those rules indelibly in your group’s knowledge
  • Add new ways to challenge your players
  • A hook to make interesting encounters
  • A little bit of world building realism added, if you like

Underwater Encounters

Ok. Here are some ideas I have on how we can use sumps to make encounters interesting and challenging for our adventures.


A rather abstract idea, but in most game system rules for drowning, time is the enemy.

Either you must count seconds and minutes, creating a fantastic deadline scenario, or each time interval (turn, round, minute) a character must roll dice to resist. Roll enough times and it becomes a juicy push your luck game.

Either way, we want to design encounters that eat up time.

d6 Time Suckers

  1. Length of passage
  2. Difficult terrain that slows movement, such as strong currents or narrow spots
  3. Forks like an underwater maze or options that create regressive loops. “Oh now, I’m back at the split again!”
  4. Hints of treasure that make greedy characters begin searching
  5. Actual treasure that is tough to get at, like a sword wedged in a crack
  6. Magical slowness


Who says you need to fill a sump up with nice, clean water?

When one considers water closet plumbing…. I mean, humanoids and tidy monsters need a place to…. Ok, backing away from that one.

But what about other interesting liquids?

d6 Liquids to Put in Your Sump

  1. Acid β€” for dramatic continual damage effect
  2. “Magic pool” type water β€” use any spell for effect, for example
  3. Ink β€” zero visibility, blue to make PCs look like smurfs πŸ™‚
  4. Slush β€” low visibility and difficult movement
  5. Oil β€” great for weapon health but bad for gripping
  6. Mercury β€” toxic, opaque, difficult terrain

Note that some liquids have by-products, such as gases.

If you get covered in oil, you’re not flammable per se. But vapour coming off certain liquids is flammable, explosive, or toxic.

So think secondary effects when filling your dungeon sumps.

Water II?

Who says you need to fill a sump up with nice liquid?

Especially at high levels or for characters with special powers, we can up our game a bit here.

d6 Non-Liquids to Put in Your Sump

  1. Ice
  2. Fire
  3. Swarms of insects or vermin
  4. Fungi, mould, oozes, and slimes
  5. Sharp jutting crystals
  6. Exploding mines or traps

You can combo non-liquids with liquid, too.

Add swimming monsters, floating hazards, and changing-state materials.

Beat the Slump With Sumps

Hopefully this little terrain feature gives you ideas for potential encounters and adventures.

Put sumps in diabolical locations.

Fill them with strange and wondrous things. Surprise your players with nasties and boons.