The USN began construction of Byrd Station at 80°S, 120°W with traverses out of Little America V in 1956-57 during Deep Freeze II. These efforts were in advance of the International Geophysical Year (IGY; from July 1957 to end of 1958) that saw several exploratory overland traverses with tractor trains (Sno-cats and modified bulldozers). Starting in January 1957 (pre-IGY) Charles R. Bentley led a traverse from Little America V to the new Byrd station along the route blazed by United States Army engineers a few months before (the Army-Navy Drive). His team conducted measurements of ice thickness and of the Earth's magnetic and gravity field. The following summer season (1957-58) he led a second traverse out of Byrd Station that visited volcanoes of the Marie Byrd Land Volcanic Province for the first time. The traverse reached the Sentinel Mountains beyond eastern Marie Byrd Land before returning to Byrd Station. Bentley led a third traverse out of Byrd Station to the Horlick Mountains in 1958-59. These three traverses led to the discovery of the Bentley Subglacial Trench or Trough, a deep bedrock chasm between MBL and the Transantarctic Mountains of East Antarctica.