In Aberystwyth?
Look, the entire town is built on glacial till (right next to the architectural award in the University library is the strain gauge showing how fast the place is slipping down the hill) and as such, there are no tunnels in that horrible stuff.
There ARE abandoned lead mines up Cwmystwyth and Cwmrheidol, but those are mostly flooded and dangerous.
And then there's you, asking for help breaking into somewhere on a public forum.
Please don't do this.