That's very interesting. I'd love to hear more of the story. What happened next?
Mathematically, that would put periscope height at 50 feet below sea level. We could model it with the following expression:
-50 - 150 = -200
Basically the submarine starts 50 feet below the surface of the water, then goes down another 150 feet and the result is it is 200 feet below the surface at the end.
