Welcome to the Nakanai Mountains on the island of New Britain, Papua New Guinea. This stunning lush-green, forest-covered landscape is rich in topography with mountains rising up to over 2000m interspersed with deep cut valleys. Elsewhere it seems like the mountains have just caved in on themselves, and that’s exactly what they’ve done. These huge circular collapse features (pictured) are called dolines, and they occur when the roof of a cave collapses. New Britain is well known for these huge dolines, sometimes called mega-dolines. For cavers they are exciting as they mean there’s a huge cave system underneath waiting to be explored. During our expedition in 2006, the aim was to descend and explore the Ora mega-doline.