Strands of Life: Book 3 of the Star Walkers Trilogy. The Star Walkers Trilogy, #3

While the Star Walkers is a trilogy, each book stands alone with a satisfying conclusion. Eric Shade, a seasoned Amazon guide, purchases a 16th-century letter written by a Spanish conquistador. It reveals a set of directions an old Indian gave him as to how to reach the elusive Star Walkers. Before Shade can ever hope to find the Star Walkers, he must overcome the hazards of an impenetrable jungle, dispose of the mercenaries who are trying to kill him, and avoid a group of cannibals whose compound is close to the tunnel, a tunnel Shade must enter in order to reach the Star Walkers.
The farther Shade travels the tunnels inside Muela Del Diablo, a dangerous volcanic mountain, the more he realizes that neither the mercenaries nor the cannibals are his worst threat, for he enters a megalithic bizarre world, a world of which the Sphinx and the pyramids are merely the surface markers. What Shade discovers in a chamber inside the bowels of the mountain goes far beyond the scope of anything he had ever imagined.