The Islington Tunnel is over 1/2 mile long (960 yds / 878m) and cuts underneath Angel from Muriel Street to Colebrook Row.
There is no towpath through the tunnel so unpowered barges have to leg it through the tunnel, where the barge operators lay upside down and then "walk" along the roof of the tunnel. You can read more about it on the wiki page.
Congrats to groosome for FTF and Vuur for STF