Published this week in the Journal of Experimental Biology, Jeremy Goldbogen and Jessica Meir review the classic paper by Jerry Kooyman, ‘Maximum diving capacities of the Weddell seal, Leptonychotes weddelli‘ published in Science in 1966. Among several scientific innovations, this groundbreaking paper debuted the first time-depth recorder (TDR), a device that has revolutionized the field of marine organismal biology. The image below shows the first TDR, built to Kooyman’s design by a local watch machinist and included components of an ordinary 60-min kitchen timer (upper). This limited the recording duration of the TDR and forced Kooyman to change the TDR every hour, putting him in close contact with the seals (middle). Contemporary tag studies now include a wide range of sophisticated sensors, but still rely heavily on a TDR to provide key information on ecological context and behavior.