Papua New Guinea is one of those perfect scuba dive destinations where you can combine really good diving with some interesting land excursions.
During my stay in Rabaul I did a landtour with Rabaul-Kokopo Dive where I visited many different sites that told the rich WWII history.
During WWII Rabaul was in hands of the Japanese. During that time the Japanese build many underground buildings, forcing locals and prisoners, to build the tunnels for them. There’s even a 5 floor deep underground hospital.
A lot of that time has been preserved and is now available for viewing. There’s a museum, the PNG Club and the bigger sites, such as the hospital as well as many tanks and weapons.