It might not be the world's largest, like nearby Son Doong Cave, but Phraya Nakhon cave is pretty darn awesome looking nonetheless.
Set inside Khao Sam Roi Yot National Park, the cave can be reached from the Hat Laem Sala beach, but you have to take a boat or a 20-minute hike there.
Oh, nbd, here's the view on the way to the cave...
Once there, you'll see that the cave is split into two light-streamingly bright chambers, and there's a pavilion that was erected to honor a visiting king in the 19th century. The cave trail is nearly 1,500 feet long.
Close up of the pavilion