Beihai Tunnel is one of Matsu's signature sights, a boat tunnel carved into the granite of Nangan. The water inside is still and dark, reflecting the rock walls and giving the place a theatrical hush.
Depending on conditions, visitors may be able to walk the tunnel path or take a short boat ride. It is the best single introduction to Matsu's military landscape, because it shows how completely the islands were shaped by defence planning.