Ocean Monument

Ocean Monuments also know as Ocean Temples are naturally generated structures that appear in Deep Ocean biomes, sometimes intruding into nearby regular ocean biomes. The are inhabited by Guardians and Elder Guardians.

Structure
The monument is constructed entirely from the several variants of Prismarine, and lit by Sea Lantern blocks. The monument is large and pyramid-like, and consists of at least six rooms. One of the types of room that can spawn is a room that has wet sponge covering the ceiling. The wings are part of a giant room that makes up the bottom section of the dungeon. In the back middle of this room are 8 gold blocks encased in Dark Prismarine, the only treasure of the dungeon.

Inside the main entrance, there is a spire that goes up through the dungeon. There are three Elder Guardians in every ocean monument that acts as a boss, though it is not an official boss mob. They are located at the top floor and each of the lower wings.

The ghostly image of an Elder Guardian and the infliction of Mining Fatigue III when approaching a Water Monument is caused by the Elder Guardians within the dungeon, not by the dungeon itself.

Strategy
A good strategy for exploring the area in Survival is with invisibility and water breathing potions, paired with Depth Strider enchanted boots (only one piece of armor may be used in order to avoid Guardian attack.) Milk Buckets are also useful to combat the Mining Fatigue effect.

Trivia

 * The base area of the monument is 58×58, quite a bit larger than Desert Temples (21×21) and Jungle Temples (15×12), and somewhat smaller than the average Stronghold.