I've worked in quite a few historic buildings over the past 30 years, and underground tunnels... well they are all just myths aren't they? Escapes for Templar Knights, hiding illicit treasure, linking the monasteries and abbeys so the monks and nuns can have a good time (I presume reading books to each other).
But no, I was wrong. This one is really does have a tunnel.
https://www.rightmove.co.uk/properties/134917190#/?channel=RES_BUY