This rather obscure 1964 B & W Spanish horror production gained some interest some years ago because of an early film appearance by future Jess Franco muse Soledad Miranda (Vampyros Lesbos & She Killed In Ecstasy 1970), in addition to it being Ingrid Pitt's (The Vampire Lover's 1970) film debut.
While digging on mountain (using dynamite ?!) in Greece two archaeologists Dr. Pete Asilov (James Phillbrook) and Professor Andre (Arturo Fernandez) uncover a couple of petrified eggs in a cave. Not noticing that one cracks open and a reptile like creature emerges, then disappears. Later Andre's partner Dorman (Jose Bodalo) shows up with his girlfriend Sofia (Ingrid Pitt), his associate Strevos (Francisco Piquer) and his driver Andre (Antonio Casas). Dorman has half of a map then when matched up with the other half that Asilov shows that a treasure may be buried in a mountain. Later a mummified body (described as a "neanderthal man") is found along with some human bones. The group stays at a nearby villa with Andre's niece (Soledad Miranda) and an elderly housekeeper Calliope (Lola Gaos), who tells them about the ancient superstition of a monster guarding the mountain.
While alone in the cave Strevos is attacked by an invisible monster and pretty graphically (for the time) killed. Soon the bodies began to pile up and the group realizes its being stalked by an invisible "dinosaur" thing. Andre sacrifices himself while trying to seal the cave up with dynamite (Ahh..that's why they had it.) and the housekeeper is killed while fetching water at a well. The premise of the movie does sound cheesy in a way (and it is), but it still seems to work in a rather odd/low budget way. The monster is never really seen except in some fleeting frame shots and at the end when its burned in a car, its seen in silhouette and a roar is heard throughout the movie as it approaches. The invisible concept was most likely done as a budget saving device, but there some kind of clever concepts used as a result of that. When axes are thrown at the creature and land with a thud, they hang in the air while the screaming roar is heard. At one point flour is spread on the ground to follow the monster's footsteps as they appear and there are some neat POV shots as it stalks thru the villa. As an added plus Ingrid and Soledad get to do a pretty cool go go dance together at one point.