No. of Recommendations: 2
I'm also no expert on alien submarine technology, but I wouldn't say it's physically impossible for something to erroneously appear to be as large as a football field travelling at 200 mph under water.
Very possible. There are oscillating layers deep in the ocean (waves), some based on thermoclines,and the speed increases with depth. If you get a rogue or just odd wave, it can move up higher and could be as large as a football field, but most of what you are detecting is movement up and down. A tsunami has speeds up to 600 miles and hour, travels in a short long wave, and then piles up near shallows. If you are in the right spot so that your detector is reflecting off the face of one of those waves, it will look like it's traveling very fast.
