Umboi Island, 50 km in its longest dimension, is the largest of the volcanic islands off the north coast of New Guinea. Dominantly basaltic-andesitic Umboi is comprised of several coalescing stratovolcanoes that are cut by a large 13 x 17 km caldera whose walls rise up to 1 km above its floor. The caldera is widely breached to the sea on the NE side and contains three youthful post-caldera cones with summit crater lakes, Talo, Soal, and Barik. The largest of these cones, 1494-m-high Talo, displays several thermal areas and has satellitic cones on its flanks. No historical eruptions have been reported at Umboi, but activity of the post-caldera cones is thought to have continued until the last few hundred years (Johnson et al., 1972).