It is a hidden world of darkness and intrigue.
In a quiet corner of the Upper Swansea Valley lies the deepest cave network in the whole of the UK.
Amongst cavers, Ogof Ffynnon Ddu (Cave of the Black Spring) is famous... but it's a place few people have ever explored.
The cave is closed to the general public, so I hooked up with South Wales Caving Club to explore its dramatic depths.
The cave goes down some 275 metres, which is three times the height of Big Ben
It's one of the longest caves in the country, about 50 km of passage that we know of and people are still discovering bits and pieces as time goes on..
The first few hundred metres are wide and tall, dispelling the myth that caving means wriggling through tight spaces, and as we venture further in we pass some weird and wonderful sights.
At the heart of the cave lies the 'streamway' - an underground fast flowing stream which it is sometimes possible to navigate.
The walls are jet black, and the water deep. I feel like i've entered a secret underground world...
Once we're out of the water, we have a few tight spaces to squeeze through before finally making it back to the surface.
You can watch Nicola's adventure here: