On the other side of the valley from Penwyllt, almost opposite the cottages, is the entrance to Dan-yr-Ogof. This is a fine and extensive system, and the river which flows out of Dan-yr-Ogof initially leaves the surface on the Black Mountain, more than two miles from where it can last be seen in the cave.
The entrance series of Dan-yr-Ogof is a show cave, but a 10 minute walk through the show cave leads to the start of the 'cavers cave'. Wetsuits are recommended as the Lakes and the Green Canal are cold and deep. Water levels in the Lakes especially are known to be very variable and can rise with little or no warning.
Control of caver access is through the DYO Cave Advisory Committee on behalf of the Show Cave owners. All parties (max 5 pers) in the cave must have a Dan-yr-Ogof Leader with them (called a warden) and have recognised caving insurance - but see the DYO CAC site for up to date access details.
And you will have to dodge the dinosaurs on your way in . . .