If you’re going to use a natural cave as a dungeon setting, don’t let it be “just another cave.” Don’t make yet another featureless, gray-stone cavern. Do some research! Make it exciting! Make it memorable! Work those set pieces!

Original Link
Izi Travel: 15 Most Beautiful Caves

There are some great photos here, and more are just a Google Image Search away. And if you want to more information about the individual caves, I’ve gone ahead and linked relevant Wikipedia (or other) articles where I found them.

Georgia – Deepest cave in the world
Wikipedia: Krubera Cave

Wikipedia: Melissani Cave

France – caves famous for French Resistance
Le Jorjane: Choranche Cave

Kentucky, USA
Wikipedia: Mammoth Cave

“Borneo / Deer Cave”
(Unable to find a good link)

Kentucky, USA
“Dixon Cave”
(Unable to find a good link)

Alabama, USA
National Geographic: Iron Hoop Cave

New Mexico, USA
Wikipedia: Carlsbad Caverns

Wikipedia: Son Doong Cave

Wikipedia: Grjotagja Cave

Wikipedia: Kungur Ice Cave

UK (actually contains several cave systems)
Wikipedia: Yorkshire Dales

Now, go out there and do some caving! Because I guess it isn’t called spelunking anymore? Kind of a shame — I don’t know why, maybe it was derogatory?