Within its bowels is a cave called Phra Non.
It comes alive from between 17:30 and 18:30 each evening, as thousands upon thousands of bats flap on out for the night's exploits. They pour out in a line that looks a bit like pollution puffing from an environmentally-unfriendly factory, as the sun sets behind nearby hills.
If you've never seen a bazillion bats fly out of a cave, then this is worth doing, but if you have, we'd suggest you spend the afternoon eating in Ratchaburi instead as it's a bit of an expensive charter to get out here.
You can get here by taking a yellow songthaeaw from Rot Fai Road, but you'll most likely have problems getting back into town after dusk if you don't arrange for a tuk tuk or motorcycle taxi to pick you up.
Most visit by chartered tuk tuk -- Numsin Hotel can arrange one for 600 baht -- ask them to throw Khao Ngu in as part of the deal.
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