Byloth is a massive underwater city on the flooded planet of Abylos, and was the first underwater city to be completed. Skyscrapers composed of bluesteel reach toward the surface, fish and other sea life curling around them, while a massive network of tunnels connects to the various buildings in the city and reaches far below ground. There are tubes connecting the city's buildings to each other as well. Most of the aboveground buildings are homes or workplaces; stuff like shops and other public places are built into the tunnel network for ease of access (with the exception of Veldor's coffee shop, as he preferred the view aboveground). The city gains power from massive solar panels with bases on the ocean floor around the city, like massive spires reaching above the surface- a massive kelp forest also grows from the spaces between Byloth's buildings and glass tubes. Food is gathered from coral reefs around the city, as well as farms all around the outer edges. The city is a generally pretty happy place, though not exactly peaceful- it's about as bustling as an American big city.
Several other ocean cities like Byloth have formed since construction on the original began, and they're all interconnected. Some use the tube and tunnel system Byloth does, while others have that classic "ocean dome" look. Byloth is the biggest of them.
Byloth and its smaller cousins are near where the shores of an old continent are, just after the dropoff into open ocean, and before the big melt, you could see the tops of Byloth's then-rather-unpolished skyscrapers poking out of the water. It and the subcities were built to double as an apocalypse shelter, since by the time people started building the city, it was already known that the world as they knew it then was headed for destruction. The ice caps were already beginning to shrink slightly by then. Byloth itself was near finished by the time the big melt happened- the tunnels were complete, and there were just a few buildings waiting to be completed and drained by then. Other cities were roughly halfway done, including the first ocean-dome town. So several populations of sentient species were moved into the city and what was finished in the subcities. Some didn't make it, unfortunately, like the desert-faring Najalos. San and a couple others were the only ones to make it to the city.