You can build down to where the world stops, but you can't use the terraformer on the bottom of the world, so you either have to build floating structures (which I can't remember if you can even do down there) or build something just above where the world stops and then build down from there (vertical tubes/rooms etc).
It would be nice if the devs would make an entrance to this in the koosh zone, underwater island area, between the sparse reef and mushroom forest (found a big hole there), and the deep grand reef (expanding this down and out further from the aurora/starting area. By going in through the caves (marked by a geyser nearby) you could follow the lava creek/river down to the first cavern (the upper one). Then in the center of that would be another type of geyser (a set of them in a ring sort of shape) that marks the set of caves that lead to the lower cavern. This would let the devs make the lower cavern be set closer to 2k down and the world bottom at around 3k (giving us a 250 - 500m solid base to build on.
It would be nice if the devs would make an entrance to this in the koosh zone [...] the deep grand reef
There actally is an entrance south to the Koosh Zone (north of the Aurora's destroyed bow) and in the Deep Grand Reef (at the deepenst point more or less directly below the Kelp - Grass- Grand Reef junction) There are more access points in the map, but I have only found those two. Both entries eventually lead to the lava castle but I highly doubt the topology of the zone stays even close to how it is now.
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There actally is an entrance south to the Koosh Zone (north of the Aurora's destroyed bow) and in the Deep Grand Reef (at the deepenst point more or less directly below the Kelp - Grass- Grand Reef junction) There are more access points in the map, but I have only found those two. Both entries eventually lead to the lava castle but I highly doubt the topology of the zone stays even close to how it is now.