According to Oxalis Adventure Tours, only 70 bookings were available for the rest of this year.
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The five-day, four-night tour comes with a team of safety experts, guides and porters, and requires high fitness levels.
The tour runs once a week with a group of 10 adventurers.
Son Doong, along with Ha Long Bay, has been named among the top 10 "most breathtakingly beautiful places to visit on the planet" by travel website GlobalGrasshopper.
