and is rocky with steep cliffs on the east side. The thin stretch of rock that used to close off the crater collapsed in 1780, admitting the sea through a 100 m channel; the entrance is only a few meters deep, thus allowing only very small ships or boats to enter the crater. The interior basin, 1 km wide and 50 m deep, is surrounded by steep walls up to 270 m high. There are active thermal springs.

The island is antipodal of Cheyenne County, Colorado,