Hidden behind a railway line is a hidden passageway that tourists in Keelung wouldn’t think to wander through. Only a small, nondescript sign hanging above its short entrance hints at its significant history.

The tunnel was built by the Japanese in 1903 (Taiwan was under Japanese rule from 1895 to 1945) and is now referred to as the “100-Year Bomb Shelter.” This portal into the city’s underbelly was a refuge for those seeking shelter from the bombs dropped by American forces during World War II.

In addition to acting as a long, narrow bomb shelter, the tunnel was also used as an ammunition depot. Now, it serves a far less risky purpose. As it’s concealed within a bustling area, locals have begun using it as an underground shortcut while going to-and-fro. It lets them enter the hillside from one neighborhood and pop out in another, without having to deal with any crowds. Some guides have also started including it as a quirky, off-the-beaten path stop on tours of the city.