A Hang Son Doong explorer navigates a plant-covered cavescape. Phong Nha Ke Bang National Park, Vietnam. (Carsten Peter/National Geographic/Getty Images)

Almost six miles long,650ft-wide and 550ft-tall, in Quang Binh province in Vietnam is so big, it can fit a 40-story skyscraper. But the cave, which was created 2-5 million years ago by river water eroding limestone underneath a mountain, actually remained undiscovered until 1991 and unexplored until 2009.

Now, tourists can explore the mythical cave, considered the world's largest, thanks to tour company, .

Oxalis Adventures, the only operator with permission to run tours to the Son Doong, has been to ensure a sustainable future for the cave, according to the Guardian. For 2014, they are able to take 224 tourists on 6-day tours of the cave, which is inhabited by60-foot stalactites and dotted with rare "cave pearls,"formed over hundreds of years when dripping water creates layers of calcite that build up around grains of sand.

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On each tour, the company can take a small team of up to eight guests to trek deep inPhong Nha Ke Bang National Park, cross stunning river valleys, and see the incredible cave formations unlike anywhere else in the world.

But the tour, which the company has described as "more like a mini-expedition" isn't for the faint-hearted. The through dense uninhabited jungle, before you rappel260ft down to its entrance, according to the Daily Mail.

Although likely not very difficult for experienced cavers, the in the cave, reported Caving News.

UK caving experts accompany each tour of Son Doong Cave, according to Oxalis. Ropes, safety harnesses, lights and helmets are necessary andare also provided. The tour starts at $3,000 per person.

First discovered by a local man in 1991, the Son Doong cavebecame known worldwide in 2009 when it was led by Howard Limbert and his wife Deb Limbert, guided by local resident Ho Khanh, according to Thanh Nien News.

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