Oxalis, a major adventure tour operator in the central province and the only licensed to conduct tours to world's largest cave Son Doong and the well-known En Cave, welcomed 6,000 Vietnamese nationals last year, increasing thrice against the previous years.

The total number of cavers, however, was down 50 percent from the previous year to 7,000 as inbound tourism plunged due to travel restrictions.

Of the 7,000 visitors, 1,000 were foreign cavers who had entered Vietnam prior to the government halting international flights in late March last year to curb the spread of the coronavirus.

With international flights and foreigner entry remaining suspended except for special cases, local businesses are focusing on boosting domestic tourism.

Currently, around 185 slots remain available to tour Son Doong in 2021, and 287 Tu Lan tours are up for grabs, according to Oxalis.

Son Doong opened to tourists in 2013. It comprises at least 150 individual caves, a dense subterranean jungle and several underground rivers. British magazine Conde Nast Traveler named Son Doong one of seven must-explore wonders of 2020.

Tu Lan, a popular setting for film shoots, was discovered in 2010, with two entrances stretching 2.2 kilometers. Tourists usually explore Tu Lan along with other caves in the system, including Chuot, Hung Ton, Ken, and Kim.

To ensure safety, the company does not receive visitors from pandemic-hit areas as recommended by the Ministry of Health. Cavers joining the tour are required to submit medical declarations.