Knytt Levels Set Underground

I'm looking for some recommendations of good Knytt levels that are set mostly underground. Specifically, I'm looking for underground levels that still have a lot of variety in their environments, or have setpieces that you wouldn't expect underground, if that makes any sense. Does anybody have any suggestions?

Off the top of my head, egomassive's A Knytt in Time has a variety of well designed underground areas. There's Kavez and Caverns, of course, but they don't really meet your requirements.

I'll see if I can come up with some more...

Infestation by Momruoy
The Healer by Pale Imitation has a few styles of underground area
Hpnagogia by plural is a bit mechanically themed but there might be something of interest
The Life Ruby by Ranger X is mostly brickwork but there's one very good cavern-esque area
Caverns Monotony by Vinterrun

Thanks! I checked out three of the games you listed (Hypnogogia, Infestation, and The Life Ruby), and of those three I think Life Ruby is the one that I'll find most helpful. Infestation just isn't what I'm looking for, and Hypnogogia, while good, is much more spare than I'd like.

Maybe I should lay out why I'm asking for underground levels. I'm working on a level that takes place mostly underground, and I'd like to see how much I can vary an underground place without violating that sense of... underground-ness. Not that I won't shy away from places that you wouldn't think to see underground! Just that I don't want such places to take up a whole level.

Am I making any sense here, or do I just sound like the guy from Hudsucker Proxy? Well, anyway, I'll check out the remaining titles on your list, although I think I've already played Caverns Monotony.

These may be a bit obvious, but the first things I thought of were Underground by Grimwit, and Underground House by Salmoneous. Underground features some fun "set-pieces" that keep even simple passages interesting. Underground House does a very good job of making you feel like you're in a quirky underground house, not simply a cave with furnishings.

Caverns by bunnrey is the ultimate underground only level.
But there's plenty more good ones like climb by AA and lava caverns by mr.affu. Look around in ks releases and knyttlevels.com you'll find them.

The link in bunnrey's thread is dead, unfortunately. I managed to find an outdated version of the level with google but haven't had any luck finding v0.8  

Re: egomassive: I only dug through my own collection of levels so I've probably missed a lot of good environmental levels off the list.

The Frozen Mountain 2 by Drakkan has a nice underground area.