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.