Colo volcano forms the isolated small island of Una-Una in the middle of the Gulf of Tomini in northern Sulawesi. The broad, low volcano, whose summit is only 507 m above sea level, contains a 2-km-wide caldera with a small central cone. Only three eruptions have been recorded in historical time, but two of those caused widespread damage over much of the island. The last eruption, in 1983, produced pyroclastic flows that swept over most of the island shortly after all residents had been evacuated.