Authorities in the central province of Quang Binh, dubbed as Vietnam's cave kingdom, are allowing travel agencies to arrange trial expedition tours inside Thoong Valley surrounded by a system of million-year-old limestone mountains and primeval forests. Quang Binh is home to Son Doong, the world’s largest cave, and UNESCO heritage site Phong Nha-Ke Bang.

Travel agencies have begun launching three-day expeditions inside Thoong Valley in which visitors are able to conquer the Hung, Tron and Thung caves, swim through a lake with emerald waters to admire the system of striking crystal stalactites and to camp in the middle of a primeval forest.

The first stop of the tour is Hung Cave.

Ngoc dong, also known as 'cave pearls,' look like eggs inside Hung Cave.

This natural phenomenon is formed over hundreds of years through dripping and creating layers of calcite that built up around grains of sand.

The next destination is Thung Cave.

To reach deep inside Thung cave, visitors must swim across a 30-meter-wide lake with emerald-green waters.

The lake is covered with a limestone system and stalactites.

The entrance to Tron (Round) Cave.

Tron Cave is half dry and half wet.

Inside the cave there are lakes and underground rivers. Visitors have to swim or stand up paddle (SUP) to reach the cave’s back door to continue the journey. The experience of swimming in a cave can be overwhelming because you don't know when it will end.

After leaving Tron Cave, visitors will see a waterfall surrounded by large rocks.

As part of the tour, visitors set up camps and go trekking in the middle of a magnificent primeval forest.

The tour is for visitors aged 15 and above and is limited to maximum of 20 people to ensure safety and avoid having a negative impact onto the world heritage site.