Acrityrda is the hollowed out innards of a giant Atlas tree that has fallen down. The tree stood about 6 kilometers tall and was the largest of them all when Terrahypt was still a part of Zerana. During Yluxa, the tree got pummeled by gravitational forces which made it tilt. When it started to decay, it fell down and slammed into the dried up sea. Now the fallen log is mostly buried underneath the Dark Keigan desert, and only a third of the entire log is visible above it. The log itself has provided a great shelter from the enveloping clouds of svibra which are dangerous to be exposed to.
After thousands of years, millions of cubic meters have been hollowed out from the log to make the subterranean world it is today. Acrityrda is now the main capital of Dark Keigan. It is not exactly densely populated because the inhabitants have made the settlements inside it into towns which only about 100 to 1000 people inhabit. But multiply that by the number of towns that exist in Acrityrda and you will get about 2 million people.
Due to the thousands of years of carving and hollowing, 98% of Acrityrda is abandoned passages and halls which inhabitants will have to navigate through to get between towns. But unlike Dark Keigan, it is considerably safer...
Only if you have enough provisioning and nerves of steel.