The Thousand Fangs Below is the penultimate episode in the “Serpent’s Skull” adventure path, which has seem the PCs trudge off into the jungle in search of treasure, after a shipwreck on a tropical island. By this time they’ve gotten hints of a larger danger lurking (possibly asleep, possibly awakening) under the ruined jungle city of Saventh-Yhi, and also suspect that a Pathfinder is in trouble somewhere in the caverns under the city. The assumption is that the PCs will mobilize a rescue and/or looting mission. Seems reasonable, though which is the driving factor depends on the PCs of course.
This scenario is mostly underground action, but it’s not quite a dungeon crawl; the under-city is more of a large sandbox with multiple factions in conflict (much like the city aboveground), and the PCs are given a lot of plot freedom in how to proceed. Of course, since this is D&D, there’s a ton of combat involved, but the lack of traditional railroaded “dungeon crawl” is nice here. Also, many of the encounters can also be solved with diplomacy or cleverness instead of just hack & slash, so extra points for that.
Even though there’s nothing here that really made me go “wow, cool”, the design is solid and (as noted) quite freeform. It’s a logical continuation of the earlier parts, and has similarities with what the PCs have faced before. That may be a slight weakness, though, as the GM needs to be careful with PCs motivations here, otherwise they might just decide to pack up and go back home, tired of fighting endless hordes of monsters in the middle of the jungle.