Buda Castle Tunnel
According to Budapest legend, the Buda Castle Tunnel is where the Chain Bridge is stored when it rains. In terms of size, this could actually be true, but the 350-metre long tunnel has a different purpose in reality. It has been connecting the Chain Bridge with the residential Buda district of Krisztinaváros since 1857, while the bridgemaster himself beavers away in a secret control room hidden somewhere around the tunnel. For most of the day, two busy lanes of traffic fill the tunnel with fumes and noise, so you may want to skip its pedestrian lane and hop through one stop by bus.