HMS Challenger recorded its deep sounding at station 225 in 1875.
Specifically, on 23 March 1875, the British survey ship was crossing the western Pacific during the Challenger expedition. At station 225, its crew lowered a weighted sounding line and recorded a bottom depth of 4,475 fathoms. A second sounding at the same location confirmed the result, making it the deepest measurement of the expedition’s three-year voyage.
The station lay between Guam and Palau, near the southern Mariana Trench. The reading did not yet reveal the full depth of the feature: later expeditions using echo sounders and improved positioning found seabed depths exceeding 10,900 metres in the nearby Challenger Deep basin.
The date and station number are useful safeguards against a common confusion. 1951 refers to the later HMS Challenger II survey, which re-examined the region and produced a much deeper measurement. The 1875 observation was nevertheless historically important because it first located the extraordinary depression and prompted later investigation.