What’s funny is you can find old maps of the Holy Land, probably from the 18th and 19th century, that show the 5 cities of the plain marked under the Dead Sea. So it can’t have been a totally new theory.
Back in the 1950s a US preacher cultivated good relations with the King of Jordan and did a bunch of dives in the Dead Sea, then wrote a long rambling (self-published I think) book about his search for the cities. Somewhere around here I’ve got a paperback of it from back then.
It’s probably based on the Old Testament. :^)