Sodom, Gomorrah, Admah, and Zeboiim were in the Valley of Siddim, and their sites are now under the waters of the Dead Sea, which formed after the cataclysm. Only the "little" Zoar site (presumably a small fort that controlled one of the important approaches or trade routes) remained visible when the Israelites returned from Egypt.
NTSA, sorted:
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.