The Japanese Cave, 4km northeast of Kota Biak, was used as a base and hideout in WWII by thousands of Japanese soldiers. A tunnel from it is said to lead 3km to the coast at Parai, but an earthquake made it impassable. In 1944, an estimated 3000 Japanese died when US forces bombed a hole in the cave roof, dropped petrol drums into it and then bombarded it from above.
From a concrete walkway, steps lead down into the spooky biggest cavern with a hole in the roof through which tree roots dangle. In and around the ticket office is a collection of Japanese and US weapons, equipment and photos.
An ojek from town costs 15,000Rp. Otherwise take a Bosnik-bound taksi and ask to be dropped at the unsigned road that leads 700m up to the cave. After heading uphill for around 300m, when you get to the top, a Japanese gun emplacement overlooks the airport. This was the focus of all the fighting.