1951 – SV HMS "Challenger II" –– Seventy-five years later, the 1,140-ton British survey vessel HMS "Challenger II", on her three-year westward circumnavigation of Earth, investigated the extreme depths southwest of Guam reported in 1875 by her predecessor, HMS "Challenger". On her southbound track from Japan to New Zealand (May–July 1951), "Challenger II" conducted a survey of "the Marianas Trench between Guam and Ulithi," using seismic-sized bomb-soundings and recorded a maximum depth of . The depth was beyond "Challenger II" echo sounder capability to verify, so they resorted to using a taut wire with "140-lbs of scrap iron", and documented a depth of .