Very Deep Lake with Partially Flooded Ancient City Beneath

For the most part, deep dark biomes will attempt to avoid being beneath water at all costs, but this is not one of those times. Not only has the ancient city failed to avoid being beneath water, but it spawned almost directly under a very deep lake in a way that has flooded a portion of it. This means that with a simple door, you can get some end-game loot with almost no risk.