When I last visited the Sandsend end was locked, but the Kettleness end was open (although requiring a journey through the shorter Kettleness tunnel to reach it)
The track between the two is very overgrown and the tunnel portal in poor condition, the seaward side is angled and looks like it may collapse within a few years.
A sizable heap of mud has washed through the roof which is held up by a metal arch, although its hard to tell how long that has been in place.

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Inside the tunnel appeared more stable, although we only ventured a little way due to time constraints.

I need to return and take more photos as this visit predated me having a DSLR.
Rikj has some amazing shots of the rest of the tunnel on Flickr