It's been brought to my attention that we don't have a Down Below page yet! So hello, here is a very rough one that is definitely missing things. PLEASE add, edit, and prune as needed.

First up: you unlock the relevant quest through a Distraction from The Watcher with the subject line “I need a favour.” You may first need to check out the “Intriguing-looking hole in the ground” in any Outpost, may first need to reach level 10, and may first need to complete some early quests.

And now for spoilers! DOWN BELOW, also called the UNDERGROUND TUNNELS, is a cool way to get from one Outpost to another, work on your unhealthy pallor, or build up Favour with the Watcher without humiliating yourself on the FailBoat. It is also the only place Retraining Personnel won't go to collect your lifeless body.

So. Miscellaneous facts and advice, which will make more sense if you've unlocked the area in-game:

If you have any questions beyond this, please go ahead and ask them in Banter and/or tack them on the end here. Good luck down there!

1)
Back when Down Below was new Contestants used to tip each other off about the current Outpost levels by posting them in story chat / scratching them into the walls as a marker for others. Unfortunately the floors change quickly enough that it's only kind of useful.
2)
I want to say two thousand but could easily be wrong.