One of Thai Buddhism's holiest sites, this temple was founded in 1624 after the discovery of a natural depression in the rock resembling a footprint, which fit with local legend about the Buddha visiting what is now Thailand. The footprint now sits under a dazzling mondap (square, spired building) and merit-makers come from all corners of Thailand to put a piece of gold leaf on it.
There are many other shrines around the grounds, including a cave with a reclining Buddha and lots of bats at the top of the hill: once you're near the summit, follow the arrows.
Down below, the Phra Phutthabat Museum (8am to 5pm Satuday and Sunday) tells the history of Buddha footprints and holds valuable temple treasures such as old pottery and a set of statues for a rain-making ceremony.
Wat Phra Phutthabat is 20km from Lopburi. Take a Saraburi-bound minivan (17B, 20 minutes) and then walk 1.5km or take a moto taxi (20B) to the temple. Or you can see it on Noom Guesthouse's afternoon tour (800B to 1250B for groups of one to five people).