Not sure how good coordinates are so follow hint.
When Sapperton Tunnel on the Thames & Severn Canal opened in 1789 it was the longest canal tunnel ever constructed in the United Kingdom. It is 2.17 miles (3.49km) in length, and today is the third longest.
Construction started in 1783 and the first boat passed through in April 1789. The last commercial traffic to use the tunnel was in May, 1911.
As early as 1916, roof falls made the tunnel impassable to normal boat traffic.
Today Sapperton Tunnel remains impassible with a number of significant roof falls, and a complete blockage near the Sapperton end.
From time-to-time the Cotswold Canals Trust conducts an informal survey of the tunnel to assess its condition. The last of these was in November 2008. More on this later.
Big thanks to Dylan Burgess for bringing this area to my attention