Jeju uprising to Jeju from Busan and deploying police companies, each 1,700 strong, from the southern provinces of the mainland. The guerrillas retreated to their bases in the forests and caves around Hallasan, an extinct volcano and the highest mountain in South Korea. On April 29, the Korean, non-military, governor of Jeju province abandoned his post, defected, and joined the guerrillas. This caused many police officers, disillusioned by the atrocities they were ordered to commit against their own, to do the same. In response, U.S. military provincial governor William F. Dean ordered a purge of SKLP sympathizers from the ranks of the
Newberry Volcano magma chamber under the large, cauldron-like caldera at its summit. This caldera has dimensions of and formed about 75,000 years ago. Newberry Volcano first formed about 600,000 years ago, and has since been built up by several thousand eruptions. About 500,000 years ago, Mount Newberry attained an elevation of . Multiple eruptions created the caldera, but the main caldera-forming event occurred around 75,000 years ago from a major explosive eruption. It formed the crater lakes and Paulina Peak, which is the highest point on the volcano, at . As a result of its caldera-forming eruption, Newberry has a horizontal profile,