Caltiki, the Immortal Monster

In 607 AD., the Mayan civilisation abandoned the city of Tike. Seeking to know the historical reasons, the archaeologist John Fielding leads an expedition to the temple of the Goddess Caltiki. In the area, they discover a hidden pool and one of them dives to explore it. Bad idea. The poor man is stripped of flesh by an horrible creature. Nevertheless the group manages to destroy the thing and gets some sample to bring back to their Mexico laboratory accompanied by an ancient Mayan inscription found near the pool. After decryption, the inscriptions speak: the radioactivity of a running comet had the effect of making the thing grow to a gigantic size...