Nearly 80% of bookings for tours to the world's largest cave Son Doong for next year have come from Vietnamese and American tourists, according to travel agency Oxalis.
Oxalis, the only firm to offer Son Doong expeditions, said 900 visitors have booked for the tours, with 40% of them or 360, being Vietnamese.
Among foreign nationals, Americans topped the list with 330 bookings, followed by Australians and Singaporeans with 45 each.
A campsite inside Son Doong Cave, Quang Binh.
This year, 1,000 joined the Son Doong tours, including 91 foreigners.
Son Doong, part of Phong Nha-Ke Bang National Park in the central province of Quang Binh, has catapulted to global fame since it opened to tourists in 2013.
Provincial authorities only allow tours to the cave between January and August, with the cave closed during other months to allow its ecosystem to recover. Visitor numbers are also limited to 1,000 per year.
A four-day-three-night expedition of Son Doong Cave cost VND69.8 million (around $3,000) before the pandemic, which will be the same price for 2023.
On the expedition, tourists can explore the cave together with a specialist, traverse unique underground rainforests, wade through rivers both above and below ground, and scale the 90-meter "Great Wall of Vietnam" at the end of the cave with ropes and ladders.
In addition, they would be treated with three freshly cooked meals per day, plus snacks and sleep in some of the world's greatest campsites.