Deep Water

From publisher blurb:

Mini-Dungeon for 4–5 characters of levels 4–5

The secrets of spell-storing gem fabrication remain closely guarded by the deep gnomes, but the most important ingredient is immediately obvious: raw gemstones.

Recently, one such gnomish-held gem mine deep beneath the surface was overrun by monsters and the erstwhile owners are desperate to reclaim it.

The deep gnomes are offering a magic gem each to any adventures who can clear out the monsters, but the air is thick and hard to breathe, and even the remains of the fallen may hold dangers. Any would-be saviors or greedy mercenaries who don’t use their wits will find themselves in Deep Water!