Go down into the Crystaline Crevass full of spiders and click on the "wriggling dwarf sized cocoon", then talk to the freed dwarf. Do this six times. The cocoons respawn once every 3 minutes or so.
The spiders don't see stealth/invisibility and opening the cocoons doesn't break stealth or invisibility, but you have to talk to each freed dwarf so you have to break invisibility for that.