Hai Van Tunnel

The Hải Vân Tunnel, the second longest tunnel in Southeast Asia (after Singapore's 12.46 km (7.7 mi) long Kallang-Paya Lebar-Marina Central Expressway tunnel) and the longest in Vietnam at 6.28 km (3.90 mi), lies on Highway 1 between the two cities of Da Nang and Huế in central Vietnam.

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Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).

Other tunnels in Vietnam

Delaware Aqueduct United States · 137 km Päijänne Water Tunnel Finland · 120 km Orange–Fish River Tunnel South Africa · 82.8 km Ishikawa Tunnel Japan · 78.5 km Brenner Base Tunnel Austria · 64 km Mont d'Ambin base tunnel France · 57.5 km Gotthard Base Tunnel Switzerland · 57.09 km Hong Kong–Zhuhai–Macau Bridge China · 55 km