A steep escalator takes you into a rectangular tunnel, where you can walk beneath the Maas River while boats sail above and cars whizz by in an adjacent tube. The strangely shaped tunnel was the world’s first rectangular tunnel constructed via the immersed tube method.

The Maastunnel is also the Netherlands’ first car tunnel, and one of the oldest tunnels in the country created via the immersed tube method. Parts of the passageway were built on a dry dock elsewhere, then sunken into a trench in the riverbed.

Construction on the tunnel began in 1937. It finally opened in 1942 in a secret, unofficial ceremony to keep the Nazis away. Those same Nazis soon found out about its existence and placed explosives there in 1944, but presumably the Dutch resistance sabotaged their triggers, which is why you can still pass through and be amazed by the passage today.

The underground part of the tunnel is about 3500 feet long, and the lowest point is estimated to be about 65 feet below sea level. You can find the entrance next to the ventilation buildings, once of which once held a laboratory to test the air quality.