Tunnels of the Deep: Book 2 of the Star Walkers Trilogy. The Star Walkers Trilogy, #2

While The Star Walkers is a trilogy, each book stands alone with a satisfying conclusion. Most people stay clear of the uncharted regions of Amazon and its perils, yet much like a nail is to a magnet, Dr. Eric Shade is attracted to the lure of ancient lost cities, Inca gold, and the age-old legend of the Star Walkers, a mysterious group of pale-skinned people the Indians claim are gods. Having served a stint with the Special Forces, Shade has learned to survive in the maleficent environment of the jungle, but even his skills as a tracker and guide are tested when he agrees to go into the depths of the Amazon to retrieve a hoard of silver.
On his quest for the silver, Shade and a beautiful woman companion enter a labyrinth of tunnels that leads them to a fabulous ancient city, a city that has eluded archeologists for centuries. There they meet two Indians who take them through a hidden passageway to a utopian village where a tribe of mysterious people live. During their stay, they convince Shade that the Star Walkers are more than just a myth.
According to the natives, the Star Walkers had befriended their ancestors thousands of years ago. Although these Indians don't know where the Star Walkers had come from, they claim a group of pale-skinned men descended upon their ancestors in shimmering ships that supposedly had come from the sky. Shade is hard pressed to believe that part of the story, but one thing becomes abundantly clear. Someone had to have taught these natives how to travel through tunnels in total darkness and how to concoct potions and elixirs that contain superhuman powers.