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The dramatically geometrical entrance to one of Phong Nha-Ke Bang's caves.

A paddy field in the foothills of the huge limestone peaks of Phong Nha-Ke Bang.

Rivers, both above ground and below, have helped shape this vast limestone plateau into unusual karst formations.

There are an estimated 300 caves and grottoes at Phong Nha-Ke Bang.

Colourful lights illuminate the caves that are open to tourists.

Enormous pillars like this form inside the caves after millions of years of mineral build-up deposited by water droplets.

These steep hills have been carved from limestone by erosion and weathering over millions of years.

Some of the caves in Phong Nha-Ke Bang have dried up, but originally underground rivers ran through here creating this cave.

Tourists are able to take boat rides into some of the caves that are most easily accessible from the water.

Boats lined up ready to take tourists into the caves of Phong Nha-Ke Bang.

Karst hills emerging from the flat paddy fields. The circular lake in the middle of the field is a bomb crater, left over from the Vietnam War.

Impressive stalactites hang from the cave roofs, formed by minerals building up over millenia.

The unusual karst landscape of Phong Nha-Ke Bang National Park. Erosion and weathering over hundreds of millions of years have created these interesting formations.

The busy bay at Dong Hoi. Soon, work will begin on building an international seaport at Dong Hoi harbour.

Fishing boats in the Nhat Le River. Fishing is still a vital industry in Dong Hoi, providing for many of the city's residents.

Soon, traditional fishing boats such as these will be replaced by cargo ships in Dong Hoi's harbour.

Fishing is still an important industry in Dong Hoi, though it may soon be overtaken by international trade.

Highway 1A covers the length of Vietnam, passing through Dong Hoi at its mid-way point (marked).

Dong Hoi has recovered well since the Vietnam War, and this pretty city is becoming a popular tourist destination.

The railway connects Dong Hoi with Hanoi in the north and Ho Chi Minh City in the south, making it a logical choice as a central seaport.

Another new industry in Dong Hoi is tourism; visitors are drawn to the beautiful beaches that lie either side of the Nhat Le River.

View of Dong Hoi from Bao Ninh Island. The city is sandwiched between the Nhat Le River, the South China Sea and the mountains of inland Vietnam.

Da Nang (shown) is currently the busiest port in Vietnam, though Dong Hoi will soon rival it, as huge investment promises to turn the city into an international seaport by 2020.

The train station at Dong Hoi. The city has long been at the centre of transportation networks in Vietnam.

Dong Hoi is one of the major stops along the Vietnam Railway. The city will soon become even more important, as huge investment will turn it into an international seaport.

Dong Hoi airport links the city with the major city of Hanoi in the north of Vietnam.

Some of the ruins in Dong Hoi have been left in memorial of the Vietnam War.

A Vietnamese flag marks the line of the 17th parallel, the demarcation line along which the country was divided.

A bridge crosses the Ban Hai River connecting the north side of the Demilitarized Zone with the south side.

The brand new Sun Spa Resort provides luxury for visitors to Dong Hoi's picturesque beaches.

This monument at the 17th parallel depicts a mother and daughter staring across the Ban Hai River, hoping to be reunited with family stuck on the other side of the divide.

The beaches around Dong Hoi attract many tourists from around Vietnam.

One of the many bunkers that still remain in the DMZ. Tourists are now able to visit this area, more than 30 years after the war ended.

A monument on the banks of the Ban Hai River commemorates the reunification of North and South Vietnam in 1975.

The bridge over the Ben Hai River joins the North and South of Vietnam.

1968 - The Ben Hai River marked the centre of the DMZ. This was one of the most heavily bombed regions during the Vietnam War.

Ruins of the Tam Toa Church in Dong Hoi; one of many buildings destroyed by bombs during the Vietnam War.

This map shows the Demilitarized Zone in grey, either side of the Ben Hai River, dividing the country in two.

A memorial arch to Ho Chi Minh at the entrance to a bridge across the Ben Hai River, that demarcated the division between North and South Vietnam.