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 5,663 fathoms (33,978 ft; 10,356 m).[citation needed] The depth was beyond Challenger II's echo sounder capability to verify, so they resorted to using a taut wire with "140-lbs of scrap iron", and documented a depth of 5,899 fathoms (35,394 ft; 10,788 m).[16] In New Zealand, the Challenger II team gained the assistance of the Royal New Zealand Dockyard, "who managed to boost the echo sounder to record at the greatest depths."[16] They returned to the "Marianas Deep" (sic)[17] in October 1951. Using their newly improved echo sounder, they ran survey lines at right angles to the axis of the trench and discovered "a considerable area of a depth greater than 5,900 fathoms (35,400 ft; 10,790 m)" – later identified as the Challenger Deep’s western basin.