Blue Holes

The ocean monument is the only challenge underwater, and making another challenge with a structure would be redundant. The ocean is full of mystery that has yet to be discovered by anyone in real life, so why not add a touch of that undiscoveredness to Minecraft? Meet the Undertow! The undertow lives in a new biome called the blue hole, a waterlogged version of the dripstone caves along with coral, sea life, and the undertow. Unlike most underwater caves, you wouldn't have a sort of night vision while submerged, and the only light sources would be natural sea pickles or putting one in your offhand. The undertow is a ghostly, translucent creature that looks similar to the blaze and breeze in that it has a head inside something swirling about. It drops tropical fish, cod, and a rare chance to drop the heart of the sea. It can only harm you by pulling you down to the bottom faster than magma can, making drowning all the more risky. Now, why would you ever want to go there? Because of scattered and buried treasure chests of course!