I heard about this 30 years ago.
There has been a FRESH WATER PEARL industry in Bahrain for eons. The fresh water rain in Yeman (some of it) washes down through the sand and travel across the Saudi countryside through these underground river beds and comes out at the BASE of the Saudi gulf-side land into the so-salty gulf. It's fresh water.
The Bahrainis had pearls there for years, FRESH water pearls, that is, ODD sized and shaped. Cost a friggin' fortune because the awl bidness is killin' off the pearls. Imagine, Arab pearl divers, all men, of course.
The gulf used to have hamur (big white fish) and TONS of delicious shrimp. The Saudis used to build these triangular fish traps and just let the tides do their "fishing."
I didn't think there was any question but that the desert once was awash. There are many fossil beds throughout the desert which point to major water-course events.
Maybe now they are getting to be able to map them more accurately.
Hamur, or Hamoor (هامور) aka "greasy grouper", is one of the most delicious fish I've ever eaten. Always on the menu in restaurants in the Gulf countries, or at least they used to be. You got my mouth watering with that reminiscence.