Cave diving may be the most dangerous of adventure sports. But it's also an area where true exploration, going where no one has gone before, is still possible. This Australian team, led by diving physician Richard Harris, is searching for the source of a river that emerges from a cave on New Zealand's South Island. While in pursuit of the waterway's origin, diver Craig Challen set a new depth record—194 meters or 636 feet. See how the divers deal with the extreme and sometimes deadly conditions in this video.
- Nat Geo Expeditions