A group of Holocene volcanoes in central Urup Island is named after its most prominent volcano, Kolokol. Berg and Trezubetz volcanoes, flanking Kolokol on the NW, have breached summit calderas partially filled by lava domes. Trezubetz, whose name means "trident," has an eroded crater rim with three large peaks when seen at sea from the north. Kolokol volcano rises to 1328 m and is sometimes known as Urup-Fuji because of its symmetrical profile. The crater of Kolokol is not well preserved, but the volcano displays no evidence of glacial erosion. Several lava flows originate from Kolokol; one of these extends almost to the Sea of Okhotsk coast. A viscous lava flow armoring the SE flank is probably the most recent from Kolokol. Borzov volcano, the oldest of the group, lies to the SW of Kolokol. Eruptions of the Kolokol volcano group have been observed in historical time since the late-18th century. Berg volcano has been most active, but Trezubetz erupted in 1924.