Falemauga Caves are a series of lava tubes in the central ridge of Upolu. They are dark, uneven, and much less manicured than the island's beach sights, which is exactly why they are worth seeking out if you like a rougher edge to your day.
Go with a local guide, bring a torch, and wear shoes that can handle sharp rock and mud. The caves are not a casual flip-flop detour; they are a reminder that Upolu is a volcanic island first and a beach destination second.