The “great out there”. Space. The Abyss is a bottomless, surfaceless and water-filled void that surrounds the world of Dunia. The void has no end, no matter which direction you travel, or how far, you will never reach an end. It is the infinity beyond the world-dome.
The people of Dunia found out relatively early in history that the world was flat and covered by an energy dome. The world just ends at the dome and the dome is impenetrable. Outside the dome looms an impenetrable darkness. At the time of this discovery they knew nothing of the Abyss. Just that there was "something" out there.
The Abyss does not have gravity, unless you're near a floating island, and once the Dunians figured out how to pass the dome and start exploring the Abyss, the submarines that were built for exploration were built taking this in mind. The Abyss is mostly empty water with occasional floating landmasses, referred to as islets. While visually and texturally similar to water, the abyssian waters exhibit different properties than that of Dunian water.
One does not go swimming in the Abyss and live to tell the tale. While the Abyss, peculiarly, have little water pressure (equivalent to about 15-20 meters depth on Earth throughout, with slightly higher pressure near large islands), it is freezing. Depending on location, temperatures range from -5 to -60 degrees Celsius, yet the abyssian water doesn’t freeze solid unless it comes into contact with air and settles. Horror stories of colonies that have experienced a breach tell of people freezing to death in icy water that solidifies as soon as it comes to rest, turning the base into an eerie tomb enclosed in ice. The same happens with ruptured or crashed submarines. The air cause the abyssian water to freeze once the chaos settles, leaving the submarine enclosed in a crystal of ice.