Temple of the cavern lakes: A temple both underground and underwater

This temple would generate in underwater lakes and is a two-story stone tower the top floor housing a chest. Despite it's ruined, skulk-growth infested state, the top floor remains dry, torches and all, because the first floor has acted as an airlock of sorts. Upon entry, players must fight off a few drowned that have spawned inside or around the temple. Once these are dead, players must venture into the top floor, and must be careful not to trigger a skulk sensor. If they are not stealthy enough, a series of pistons will retract, opening the walls and ceiling. This prevents and complicates access to the chest because the top floor will be flooded almost immediately and the player (if he/she didn't kill them) will get instantly swarmed with drowned. If a player is stealthy, they can use wool to silence the skulk sensor, then loot the chest and run away from the drowned before they can react.

Items the chest can contain include basic ores, slime, some fish, and a skulk sensor or two, as well as some copper and amethyst shards (plus the obligatory diamond or two).

Minecraft needs a third temple, and this one's redstone trap uses its setting rather well. Also, a structure of some kind is needed to spruce up the underwater lakes, otherwise they would get boring quickly.