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Crocodiles are famously sun-loving reptiles, so why would they live in complete darkness in cool, dank caves? And where else would they do it but in Madagascar, home to some of the most unusual creatures ever to evolve on Earth? On "Secrets of the Crocodile Caves," NOVA ventures into a wilderness unlike any other.

The program explores the mystery of the croc caves and probes other wildlife haunts within spectacular Ankarana reserve, a setting of ancient coral mountains with needle-sharp ridges and spooky caverns overlooking a lush dry forest. No other place like it exists, which is why scientists such as French crocodile expert Olivier Behra come to Ankarana to observe creatures and behaviors that are unique—and strange.

Located near the northern tip of Madagascar, Ankarana is a natural fortress that has preserved many of the island's distinctive species, which took a markedly different evolutionary path from life on the African continent, 250 miles away. The island is especially renowned for its lemurs, which belong to a suborder of primates called prosimians that are more ancient than monkeys or apes. As the only primates on the premises, the prosimians flourished, diverging into dozens of different types including the crowned lemur, which NOVA investigates in a picturesque colony presided over by a one-eyed queen.

Then there's the fossa, a lithe and lethal predator that's truly hard to describe. "The fossa looks a little bit like a cat, it looks a little bit like a dog, and it looks a little bit like a hyena," says American biologist and fossa tracker Luke Dollar. "But it's actually none of those things. The fossa is actually a really big mongoose. It's like a mongoose on steroids." Chameleons also populate the island in profusion, thanks to fruitful mating—an impressive ritual captured in fascinating detail by NOVA's cameras.

But the star of the show is the Nile crocodile, which swam over eons ago from Africa and has since adopted some unusual habits. The most puzzling is its predilection, at least at Ankarana, for caves, an unlikely haunt for a cold-blooded creature that normally basks in the sun and consumes abundant riverine fauna—including 100 hapless humans a year on Madagascar. To solve this riddle, naturalist Behra cautiously ventures into the croc's underground lair—as do a troop of curious, thirsty, and very cautious lemurs.