(VOVworld) – Central Quang Binh province earned more than 1 million USD in the first half of this year alone from tours to Son Doong, the world’s largest cave. According to Oxalis, the only operator allowed to run tours of the cave, visitors rate the tour as a once-in-a-lifetime adventure with professional tour guides and staff, who pay careful attention to the health and safety of the trekkers and provide amazing service and supplies. The tour has received a lot of positive feedback on TripAdvisor, the world’s largest travel site, the firm said. Oxalis provides tour group members with the necessary caving equipment and has also prepared a landing field for helicopters in case of emergency. In addition, the operator includes public liability insurance in the registration fees. To ensure no damage is caused to the cave’s ecosystem, the water quality of the river in the cave is examined every six months, while the campsites are decontaminated regularly.