What Waits Below AKA Secrets of the Phantom Caverns (1984)
Short review: Unofficial remake of The Mole People.
Starts as if it were a war movie. Our hero is "somewhere in Nicaragua" spying on the local soldiers. An old buddy shows up out of nowhere. The locals spot them, leading to a big battle/chase scene, with the two guys easily shooting down and escaping from countless soldiers. They get to a Jeep and drive it over a cliff into a lake. Somehow, we're told, "twelve hours later," they're helicoptering into Belize. Mind you, this Nicaragua stuff has nothing to do with the rest of the movie.
Our hero is some kind of super cave expert. He's been dragged to Belize so he can help the US military set up a special radio transmitter inside a cave. (There's some explanation for why this is the perfect place for the gizmo.) The problem is that there isn't an entrance to the place! Some archeologists are around, claiming to have found artifacts "millions of years old" (!), so they're interested in the cave as well. One of them is our pretty blonde love interest.
Anyway, something causes an entrance to the cave to open, so our antagonist, a nasty officer who has some bad history with the hero, orders the archeologists to stay out, and sends two soldiers inside with the gizmo. Before you can say "redshirt," they and the gizmo disappear. Our hero, the archeologists, the officer, and some other cannon fodder go inside to find out what happened.
After a lot of wandering around, and an attack by a reptilian monster -- we only see its huge, fang-filled head, so I'm not sure if it's a giant snake or a dinosaur-like critter -- we finally learn that there are a bunch of albino folks in pure white Star Trek style duds living in the cave, and they're not happy about the intruders.
Silly stuff, to be sure, and it takes itself very seriously, but it's moderately amusing in a 1930's pulp magazine adventure kind of way.