Underground Ocean

This movie has it's faults BUT..once it gets going, it's a pretty effective piece of work. You really get a feel that these guys are underground and discovering a lost world which nobody has ever seen before. The early scenes when the adventurers discover the underground ocean are shot and directed in an impressive fashion. The early location shots where the group first descend into the volcano are also pretty good. One to check out if you dig 1970's fantasy/sci-fi horror.