Radar Coverage
Once upon a time two commercial aircraft did a mid air over the grand canyon. My source said both were reporting being elsewhere and each was taking passengers for an off track sight see of the canyon.
Results was Congress mandate the US Air Force put in place 100% coverage of the US for ATC.
Take care not to extrapolate the US flight coverage picture to the rest of the world. I get 600+ miles between Kuala Lumpur and Bien Hoa Air Base in Viet Nam. That is a lot of water with no eye's on a scope.