One of the old Forgotten Beasts, Budam the corpse blob, she just disappeared. She lurked outside the magmaforge walls where in life she was shot down by marksdwarves and remained there for decades, just venting off noxious fumes. No one could recover her corpse so there she remained in the lake. Then I get the extremely startling discovery that a magma crab had been captured in the 3rd cavern layer platform to the magma sea, and noticed not only that - but Budam's corpse was gone and all that remained was a giant rattlesnake composed of grime and filth, that had been heavily damaged - one mandible torn off, its stinger smashed open, its body and head smashed open.
Seems like the poo snake had taken issue with poor old Budam, but that doesn't explain where her corpse went. Set on fire by the magma crab? Lost to the magma sea? It'll remain a mystery.
Also of note is that several forgotten beasts have also commenced a joint attack on the surface world, two fliers, one with webs and one with poisonous blood, and ground forgotten beast webber has also joined them. Around 30 elite warriors, most of them marksdwarves, have been tasked with repelling them.
The ground beast is breaking through the 3rd layer platform, the six-legged pangolin is flying and the blood beast is licking the honey from the former bee prisons. This should be doable. On the bright side surface construction has resumed, the wall spikes are a few years away from completion and the Fort's first siege engineers are being trained for the up and coming war.