Pulau Una Una, which consists mostly of active Gunung Colo (506m), was torn apart in 1983 when the volcano exploded. Ash covered 90% of the island, destroying all its houses and crops. Residents were safely evacuated, and many have since returned.
The offshore reefs here offer the best diving in the Togeans with schooling barracudas, Napoleon wrasse, packs of jacks and large rays. It's possible to climb the volcano in three hours, if conditions are favourable (it's highly active) and admire the awesome lava landscapes.