Hải Vân Pass
The Hải Vân Pass (, , "ocean cloud pass"), is an approximately 21 km long mountain pass on National Route 1 in Vietnam. It traverses a spur of the larger Annamite Range that juts into the East Sea of Viet Nam, on the border of Đà Nẵng and Thừa Thiên–Huế Province, near Bạch Mã National Park. Its name refers to the mists that rise from the sea, reducing visibility. Historically, the pass was a physical division between the kingdoms of Champa and Đại Việt. It is nowadays also used as a signifier of the climates of northern Vietnam  vs southern areas, where northern regions receives 4 seasons (summer, winter, autumn, spring) whilst southern areas receives just 2 seasons (rainy and dry). The twisting road on the pass has long been a challenge for drivers traveling between the cities of Huế and Đà Nẵng. Since the completion of Hải Vân Tunnel, traffic flow and safety have improved. The pass has been the scene of at least two of Vietnam's most serious rail accidents, and at least one air crash. Overview. The Hải Vân Pass crosses over a spur of the Trường Sơn (Annamite) Range that emerges from the  west and juts into the South China Sea, forming the Hải Vân Peninsula and the adjoining Son Tra Island. The pass, which once formed the boundary between the kingdoms of Đại Việt and Champa in 1306, also forms a boundary between the climates of northern and southern Vietnam, sheltering the city of Da Nang from the "Chinese winds" that blow in from the northwest. During the winter months (November–March), for instance, weather on the north side of the pass might be wet and cold, while the south side might be warm and dry. The pass is renowned for its  scenic beauty. Presenter Jeremy Clarkson, then host of the BBC motoring programme "Top Gear", featured the pass during the show's 2008 "", calling the road "a deserted ribbon of perfection—one of the best coast roads in the world." History. Hải Vân Pass has been of major strategic importance in this history of Vietnam, and for a long time represented a major barrier to any land army that attempted to move between the northern and central regions of the country. During the 1st century A.D., the Chinese general Ma Yuan (Mã Viện), after pacifying northern Vietnam, advanced south and established  the southern border of the Han (Hán) empire by setting up columns of bronze, possibly at Hải Vân. Ma Yuan also left behind some Chinese military families to hold the frontier. When the Han empire collapsed at the end of the 2nd century, the local kingdom of Linyi (Lâm Ấp), the predecessor to the medieval polity of Champa, was created by a petty frontier bureaucrat of the Han administration, probably in the area of modern Huế somewhat to the north. Transport. The pass is crossed by two main transport routes: Vietnam's main north–south highway, National Route 1, and  the North–South Railway. The road uses the Hải Vân Tunnel—the longest tunnel in Southeast Asia—while the railway hugs the coastline, passing through a series of tunnels along the way. The old National Route 1 crosses over the mountain more or less directly, climbing to an elevation of and passing south of the high Ai Van Son peak. Travel time on it can be up to an hour longer. In "Top Gear" series 12's , Jeremy Clarkson called this route "one of the best coast roads in the world." Accidents. Besides its beauty, the pass is also renowned  for its difficulty. A poem by Nguyễn Phúc Chu (1675–1725) describes Hải Vân as "the most dangerous mountain in Vietnam" (). Visibility on the pass is often reduced by the eponymous mists that rise from the sea. Along with the road's winding route through the pass, this posed a serious challenge for drivers before the construction of the Hải Vân Tunnel. The Hai Van Pass has also been the scene of at least two of Vietnam's most serious rail accidents, and at least one air crash.