The purpose of the expedition was to study the hang khe rhy cave, one of the longest caves is southeast Asia. They also wanted to measure the length of the son Trach river, which flows through the cave. the cave is deep in the jungle in Vietnam's Quang Binh province. The area around the cave is home to a great variety of rare flora and fauna. Peter and fifteen other experts had been asked to study this area by the Vietnamese government.
Getting to the cave wasn't easy. Peter and the others trekked for eight hours through thick jungle to get to the cave's entrance. Then they went into the cave . They worked underground for twelve days following the Son Trach river through the cave. Peter made careful measurements so that he could draw accurate maps later. Finally they came to the cave's exit. Peter was very excited he had discovered that the Son Trach river was 11 Kilometres long-the longest underground river in the world.