Getting to the cave wasn't easy. Peter and others trekked for eight hours though 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 though the cave. Peter made careful measure 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!