In December 1872 HMS Challenger left Portsmouth as a stripped down Royal Navy warship with 15 of its 17 guns removed to make room for chemistry and natural history labs. Backed by the Royal Society of London, and driven partly by the need to lay transoceanic telegraph cables insulated with gutta percha, the voyage covered 68,890 nautical miles between 1872 and 1876, mostly under sail with coal rationed for dredging. The crew sounded the deep with 181 miles of Italian hemp rope flagged every 25 fathoms and a Baillie sounding machine that grabbed sediment cores, and they measured deep temperatures with Miller Casella and reversing thermometers. On March 23, 1875, at Station 225 between Guam and Palau, the line ran out to 4,475 fathoms, marking the spot now called the Challenger Deep.

The human cost was steep: of 237 men who sailed, 144 remained at the end, lost to disease, rigging accidents, medical discharge, and desertion at ports near gold and diamond rushes. Scientists including Charles Wyville Thomson expected living fossils and Thomas Huxley’s Bathybius haeckelii, a slime hailed as a missing link, but chemist John Young Buchanan showed it was calcium sulfate precipitated when alcohol preservative met seawater. Instead the expedition found roughly 4,700 new species, and its findings took nearly two decades to publish.

• Pressure squeezed the glass thermometers and gave falsely warm readings, and Peter Guthrie Tait’s recalibration work produced the Tait equation still used in physics.
• Crews dragged mop heads tied to heavy planks across the seafloor to snag sea stars and corals.
• Of about 1,250 days away, 713 were spent with nothing but open ocean in sight.
• The final report ran to 50 volumes and 29,500 pages, organized from an office in Edinburgh by Thomson and John Murray.
• The ship lent its name to the Space Shuttle Challenger and to an Australian skink, Saproscincus challengeri.