The Abyss is the name given to the deepest part of the ocean in Grand Theft Auto V. In a Submarine the player can travel to extremely deep ocean depths and find fish, Shipwrecks, Airplane Wrecks and even the Sunken UFO Wreckage. The Abyss encircles the entire map and is usually located a few miles off shore. Underwater from the player's point of view, The Abyss appears as a deep dark ocean trench that you can't see the bottom of. In a submarine you can dive deep but eventually once you reach a certain depth the submarine will begin to alert the player that there is too much water pressure. If you continue to go deeper the submarine will experience a structural failure and it will implode from the water pressure, killing the player. The Abyss has really no physical characteristics other than that it is very dark, so dark it's even hard for the submarine's light to cut through.
If the player uses any sort of hack or cheat to bypass the ocean depth limit, he/she will find that The Abyss is simply a large and completely black void. The player will still be able to swim around inside of it. If the player continues swimming downwards, the water will continue getting darker and darker, until the player encounters Blue Hell. After this, he/she will get teleported to the nearest pedestrian pathway on the mainland.
Rumors of the Kraken
Throughout the internet there are many rumors spreading around that the legendary Kraken exists in the depths of The Abyss. The Kraken is a relatively new myth so there isn't that much info about it yet.