Dotted across the globe are some of the most unusual tunnels that you could ever imagine, from one being mysteriously made by giant sloths to another that has been labelled an 'entrance to hell'. Tunnels, big and small, have come a long way since the first was thought to have been built in 2200 B.C., which connected the temple of Belos in Babilonia. Then came the first-ever underwater tunnel, the Thames Tunnel, which took a whopping 18 years to build and was opened to the public in 1843.