I remember a movie that scared me when I was younger, but I can't remember the name... I need your help!

I probably saw it in the early nineties, on TV. 

A group of people go into caves and there they discover a "civilization" of creatures that leave there.

I *think* the US army was part of the movie. I also remember a lake. I think at the end the king of the creatures is on a throne.

And the part that scared me : The team that is underground enters a big room and you can "see" the creatures running from  behind a pillar (Stalagmites/Stalagtites I guess) to an other, without the  people really seeing them... Just "shadows"...

I know that's not much, but hopefully someone knows what I'm talking about!

Thanks!