So after yesterday's trip to Bletchley Park I stayed in London and hopped over to a spot not far from Tower Bridge where Marc Brunel and his son Isambard built the first tunnel under a navigable river: the Thames Tunnel. The tunnel was dug out by hand using a tunnel shield (which is the basis of all tunnel building to the present day). Workers stood inside a metal cage pressed against the undug earth and removed boards, dug in a few inches and replaced the boards. Once the digging was done the entire structure was forced forwards a few centimeters and bricklayers would fill in behind.
The tunnel has a rich and varied history and is still in use today (read the Wikipedia link above to learn more). The entrance to the tunnel was through a massive circular tube (a caisson) which the Brunels built above ground and then sunk it into place. The entrance has been closed for about 140 years and is being renovated, but I was lucky enough to be taken into it by the curator of the Brunel Museum.
The museum displays works by the Brunels and runs tours through the tunnel itself. The grand entrance hall will be reopened to the public in September. Before that here's a shot of me standing in the interior of the entrance about 15 meters underground.
Image credit: Jonathan Histed
The diagonal line on the wall is the remains of where the grand staircase came down and brought visitors into the tunnel.