The highest of the Marianas arc volcanoes, Agrigan contains a 500-m-deep, flat-floored caldera. The elliptical island is 8 km long; its 965-m-high summit is the top of a massive 4000-m-high submarine volcano, the second largest in the Marianas Islands. Deep radial valley dissect the flanks of the thickly vegetated stratovolcano. The elongated caldera is 1 x 2 km wide and is breached to the NW, from where a prominent lava flow extends to the coast and forms a lava delta. The caldera floor is surfaced by fresh-looking lava flows and also contains two cones that may have formed during the volcano's only historical eruption in 1917. This eruption deposited large blocks and 3 m of ash and lapilli on a village on the SE coast, prompting its evacuation.