The journey to explore this river system, estimated to stretch 600 meters in the Son Doong Cave in Quang Binh Province, is expected to last for a week starting Monday.

Phong Nha-Ke Bang National Park in Quang Binh Province, home to the world's largest cave Son Doong.

Dr Howard Limbert, an expert with the British Cave Research Association, who has 30 years of experience exploring caves in Vietnam, including the Son Doong Cave, said it is possible that the underground river of Son Doong links up with the Thung Cave, which was discovered in 1994.

Since Son Doong was discovered in 2009, experts have only explored its dry part and left alone its river system, he said.

The four divers that will take on the latest quest are those who joined the team to rescue the 12 boys trapped with their football coach in a labyrinthine flooded cave complex in northern Thailand in July last year.

Each of them has 13-40 years of experience in diving and exploring caves.

They will not dive together at the same time. One would go down first and swim for 200 meters, using a rope to mark the place and return before another continues from that part for another 200 meters.

The work will go on until they reach the other side of the river.

Rick Stanton, one of the divers, said that each diver will carry two diving cylinders connected with a circulation device that allows them to breathe underwater for six hours straight.

This is the same method they had used when saving the Thai boys.

The group said there will definitely be unexpected difficulties, and that how the situation develops will depend a lot on the real conditions of the river system.

If it is proved that the underground river connects the two caves, Thung will then become a part of Son Doong, raising the current total length of Son Doong from nine km (six miles) to 12.3 km and its total volume will rise from 38.5 million to 40.1 million cubic meters.

The four U.K. divers who will explore the underground river in Son Doong Cave.

Son Doong opened to tourists in 2013, four years after members of the British Cave Research Association finished their exploration and declared it the world’s largest.

Local resident Ho Khanh first discovered the cave in 1991, and rediscovered it almost 20 years later, opening it up for exploration.

The U.K.-based travel guide publisher Lonely Planet said in January that Son Doong Cave is among the best places to visit in 2019, and warns that the trip has to be booked early due to limited slots.

The government of Quang Binh Province, known as the kingdom of caves, recently raised the limit for a number of tourists allowed to visit the cave from 640 to 900.

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Source: VnExpress