EDIT: Looks like a new update went out for SteamVR on 1/3 that might have fixed the issue: "Fixed a bug where the floor height offset was not always being taken into account during room setup."
Hi all! I've gotten some common feedback recently that new Rift players are starting the game underground and can't proceed. This seems to be an issue with the SteamVR room setup. You'll need to run the room setup again, and during the step where you're asked to measure the height to your headset, place the headset on the floor (make sure your cameras can still see it!) and set the height as 0. For some reason, setting a height here sometimes just pushes you that far under the ground.
I've been told that this is fixed in the SteamVR beta branch, but I haven't yet tried it myself.