This is a dashcam video of driving under the River Thames in London from south to north, via the Rotherhithe Tunnel.
This is the entire trip through the tunnel, though there are a couple of places where some frames are dropped due to the nature of the raw dashcam footage, so the video skips a bit, but this is the full journey, with no edits. Also, it's at 4x normal speed, because that would be tedious)
The Rotherhithe Tunnel is a road tunnel under the River Thames in East London, connecting the Ratcliff district of Limehouse in the London Borough of Tower Hamlets north of the river to Rotherhithe in the London Borough of Southwark south of the river, designated the A101. It was formally opened in 1908 by George Prince of Wales (later King George V).
The tunnel was designed to serve foot and horse-drawn traffic between the docks on either side of the river. This accounts for some of its more unusual design features. The roadways are narrow, with each lane only some 8 feet (2.4 m) wide, and footpaths between 4 and 6 feet (1.2 to 2 m) wide on each side. The tunnel is shallow, with a maximum gradient of 1 in 36 (2.8%), to cater for non-mechanised traffic. It includes sharp, nearly right-angled bends at the points where it goes under the river bed. These served two purposes: avoiding the docks on each side of the river, and preventing horses from seeing daylight at the end of the tunnel too early, which might make them bolt for the exit.
Like many other London tunnels and bridges, the tunnel carries far more traffic than it was designed for. It was well-used from the start, with 2,600 vehicles a day soon after it opened — a figure which was seen as easily justifying the expense of its construction. By 1955, usage had quadrupled to 10,500 vehicles a day and by 2005 usage had tripled again, to over 34,000 vehicles a day. Read more about this on Wikipedia.