Based on Edgar Rice Burrogh's non-Tarzan novel a scientist and his assistant take a machine that drills into the earth. They end up in a lost world of some sort ran by groutesque monsters (that sound like vacum cleaners accroding to the New York Times). There, giant prehistoric birds with telepathic powers, order Mayhars to capture people and enslave them. It's up to the heroes who ended up in this mess to free them with a help of a beautiful inhabitant. Cheesy special effects. From the people who gave you LAND THAT TIME FORGOT (1975).