There's a lot of stuff underground on Archais, from svirfneblins to tyrannids, spiders, and trogs. There's also been mention of drow and linkages from azeroth. This document attempts to summarize current plans for the archais underworld.

There's a big Drow (dark elf) city underground between Dragontooth and Azeroth Keep. This city was set up long before humans made it to Archais.

In the distant past, there was also another Drow city, underground south of Azeroth Keep, between azeroth keep and fort magnesia.

At some time before humans showed up, the big Drow city went to war with the smaller southern one and destroyed it. All that remains of the southern are underground ruins and enough poison gas to kill anything that comes down that far. The invaders had a scorched earth policy.

There are lava tubes that lead to the destroyed Drow city. Once centaurs start to get close to the city, they die due to the poison gas.

Later, after destroying the southern drow city but still before humans arrived on Archais, the big drow city invaded the svirfneblin city near dragontooth. The svirfs were not wiped out, but rather retreated. The Drow also left that city in ruin and full of traps. The Svirfs are even now attempting to reclaim it.

Recently, the drow came up at azeroth, taking over the keep and using it as a staging ground to support qoorik and mastermind the false temple and invasion of pellam on sloe. Storyline on this TBD.

Note that there are a lot of other tunnels and areas underground on archais, particularly in the dark forest. The tunnel networks connecting drow should eventually link to the tyrannid/spider network, as well as the svirf city, azeroth, and magnesia.