I’m not sure when this happened, but NOAA thinks they’ve finally identified the mysterious underwater sound known as ‘The Bloop’:
The broad spectrum sounds recorded in the summer of 1997 are consistent with icequakes generated by large icebergs as they crack and fracture. NOAA hydrophones deployed in the Scotia Sea detected numerous icequakes with spectrograms very similar to “Bloop”.
You can hear the sound and view waveforms at the link. I still favor the giant sea monster explanation, myself.