This entire thread is for !!SCIENCE!! about spoilers and the inner workings of the newest (0.50.x Steam) Hidden Fun Stuff. You have been warned.There are now multiple reports of exotic spires reaching improbably high into the sky in the new (0.50.x Steam) version. Some of them are semi-stable, but as of this post the best documented case varies wildly every time you embark, with reportedly about a 50/50 chance of it even being playable; which implies some sort of un-initialized or accidentally-overwritten variable to me.
Exhibit A (/u/Sigfrial) has pictures but no save or seed
Exhibit B (/u/SilverFlame97) has pictures, a save file, and instructions; AFAIK it's the first semi-reproducible save so credit to SilverFlame97 for the effort
Exhibit C (/u/Trick_Statistician27) has pictures but no save or seed (reported not reproducible by OP)
Exhibit D (/u/Wootersagain) has pictures but no save or seed yet
Exhibit E (/u/ArchTroll ) has pictures of a collapse-on-embark variant but no save or seed yet
As far as I can tell, without having had time to Delve Too Deep much myself, the intended behavior is that the new-style spires are a bundled cable of sorts through the magma sea, with sub-conduits that have various combinations of water, magma, fire, demons, angels, and ramps. They are surrounded by rare gems, like the new "geode" features in the upper caverns, to encourage... avarice
Sometimes, like the old adamantine spires, something bugs them out, and they stretch crazily into the sky; this used to be referred to as Armok's Whisker, Armok's Beard Hair, or the Great Adamantine Space Elevator. Importantly, this seems to vary from embark to otherwise-identical embark, and doesn't show up in the terrain slope; indicating that it's due to some sort of data corruption bug. As one might expect, various combinations of suddenly-released materials, cave-ins, hundreds or thousands of Z-levels of plummeting landscape mixed with magma and water, demon pathing, etc. render these a mix of fractional FPS, near-instant death, or actual crashes.
I figured someone should start a thread to collect saves and do !!SCIENCE!! both for our own amusement and to help with bug tracking, so here we are. Post about the expected / properly working new HFS and the weird bugs are both welcome.