The developers have indicated that they are working on a different project now, codenamed lithium:
in a quote from EightyTwo on discord:
Derek and I really want to chat with you all about the state of UnderMine, Thorium, and what we’ve been working on.
The exciting news is that we are fairly deep into development on a new project and we think all of you are especially going to love it. We can’t discuss details yet but we’ll have something to share next year. For now, you can call it by its codename “Lithium” and speculate as much as you like.
We began development on Lithium a month or two after wrapping the WHIP update. It started with an entire overhaul of our engine and continued with design and planning of the major, marquee features. We’re still using Unity but both UnderMine, and especially Lithium, use a significant amount of custom code. So, maybe not so much an engine, but a turbocharger. We rebuilt the turbo… to go very fast.
During our development of UnderMine we often envisioned a more ambitious project and that project means new tools for faster, more efficient, and less error prone development. So that’s what we’re doing, we’re building those tools. As of now, we’re coming out of pre-production (draw two circles) and entering full on production (draw the rest of the owl.) Courtland Winslow, who was a contract artist on UnderMine, and who was responsible for art like the Golden Core and Din’s arena, has officially joined Thorium and will be directing the art for Lithium. Derek and I are continuing to do what we each do best. He is building ridiculously good tech, and I’m designing and writing. Everything is moving along beautifully and we can barely contain our excitement. We want to share it all with you as much as you probably want to hear it.
That means the less exciting news is that development on UnderMine has stopped. WHIP will mark its final update. This might disappoint some, but ultimately in the long run we think you’ll understand why we made this decision. We’re really happy that you all continue to enjoy the game, and that the WHIP leaderboard still fills up every day. It’s our goal to blow you away with what comes next and we hope you’ll stick around and follow our progress.
Clint (and Derek, and Courtland)