As its name suggests, The Deep is one of the deepest locations in Fisch, so it is home to several creepy, weird-looking fish. While everyone is aware of the existence of such a location, nobody knows how to reach it, so if you are one of those players struggling to find the POI, here is a dedicated guide about it.
Where is The Deep located in Fisch?
To start, head to the Castaway Cliffs at GPS (690, 135, -1693). From here, keep traveling in the Northern direction into the open Ocean until you find a small barrier that takes you to the other side of the Ocean. You can notice this transition with the help of a small fadeaway black screen.

After this small fadeaway teleportation, you will find yourself on the other side of the Ocean. Here, go straight to (228, 130, -11945) to find a Ship with the words The Deep above it. Hop inside and interact with the Cadet Shimmerfin NPC while wearing a Flipper, Oxygen Tank, Tidebreaker & Diving Gear. If not, the NPC will not hand over the Deep Survive Device, which is required to proceed down the hole.
While interacting, he will tell you about the uncharted territory below and the Deep Survive Device, which he obtained while exploring the area. He also informs you that the machine belongs to the people of the floor and might help you find the area below.

Once you have finished speaking with the NPC, go near the Anchor area of the Ship (backside of the Ship), jump straight to the ground holding a Tidebreaker to keep moving towards The Depth. Within a few seconds, you will find the town with large white domes and palaces; enter the central part to refill the Oxygen and start your journey below.
For more on Fisch, check out How to enter Atlantis in Fisch - Levers, TNT & Zeus Heart or How to get out of and leave Atlantis in Fisch