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.

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 around US$3,000 before the pandemic, which will be the same price for 2023.