+1 for earthquake dive, a couple decades ago.
Sharply sloping silt bottom off a fishing beach, south coast, Jamaica. Lots of pots, ropes and some anchored boats. At about 50', I saw a line of disturbed silt and thought that it was a rope being dragged. On closer inspection, there was no rope and the silt appeared to be disappearing into a crack. Had no idea what was happening until I had gotten back to shore.
Can't say that I noticed any noise/rumble. Probably too excited and confused by what I was seeing.