Could a whale cause this?

[Big Macca]

We've experienced a similar phenomenon when crossing the notorious Rip at the entrance to Port Phillip Bay in Australia. The bay is huge and the entrance very small with a narrow and deep "canyon" across it adding to the complexity. The currents race through and it drives the depth gauge crazy.

It's been explained to me as a result of different layers of warm and cold water being pushed through, with the colder water layers forming a mass the depth gauge interprets as solid. A similar feature can be found deep in the oceans as well, and submarines can apparently "hide" between the layers.

[clownfishsydney]

We once pulled up at a known location to dive in our dive boat. The sounder showed it to be only 10 metres deep but I knew that it had to be at least 20 metres. We went around and around and it stayed the same. One of our divers jumped over and reported that it was a huge school of Australian salmon. They were just forming a solid mass that totally fooled our depth sounder (and us).