The National Show Caves at Dan-yr-Ogof have topped a poll to find Britain's favourite beauty spots.
Dan-yr-Ogof caves were discovered in 1912
A competition run in by TV channel Five asked readers of the Radio Times magazine to vote for their "finest natural wonder".
They chose the network of caves in the Brecon Beacons ahead of attractions including Giant's Causeway, the White Cliffs of Dover and Cheddar Gorge.
The top 10 also included the lake at Cwm Idwal in Snowdonia.
Dan-yr-Ogof was chosen ahead of some of the most famous names on the UK tourist map.
The short list included the Hebridian Island of St Kilda, the largest area of freshwater in the country Loch Lomond and the Jurassic coast which spans the counties of both Dorset and Devon.
The cave complex at Dan-yr-Ogof, in the upper Swansea Valley, stretches for more than 10 miles underneath the Brecon Beacons.
The chairman of the Dan-yr-Ogof caves complex, Ashford Price, said that the attraction's victory was like "David taking on the Goliaths."
"Winning this competition is absolutely tremendous," he said.
The cliffs on the island of Staffa were not enough to sway voters
"Not just for us, but for the image of Welsh tourism in general."
Chair of the Wales Tourist Board, Jonathon Jones, added: "We should not be surprised given the quality and popularity of this great example of natural history.
"This well-deserved award will, I am sure, not be the last."
First Minister Rhodri Morgan said the caves were "endlessly fascinating".
THE TOP 10
1. Dan yr Ogof
2. Cheddar Gorge
3. White Cliffs of Dover
4. Giant's Causeway
5. Jurassic Coast
6. Loch Lomond
7. Cwm Idwal
8. The Island of Staffa
9. St Kilda
10. Lundy Island
He said: "The public have voted very well indeed and very wisely.
"What they have done is recognize what we in Wales have known for a long time - that Dan-yr-Ogof caves are one of the most amazing natural phenomena.
Mr Morgan added that he hoped that the result would help draw more visitors to the caves.
Each of the finalists was featured in a programme called Britain Finest Natural Wonders broadcast on Five on Monday.
The programme counted down to the naming of Dan-yr-Ogof as Britain's "favourite natural wonder".