The most accessible part of the area's immense 25km limestone cave complex is the seven-chambered Callao Cave. The cave is reached by 184 slippery steps. Several sinkholes illuminate the cavernous chambers, the largest of which houses a little chapel. Compulsory guides (P100) are supposed to protect the cave from vandalism.
To get here, catch a jeepney (P50) from Don Domingo Market, just north of the city, or make the return trip by tricycle (P500, with waiting time).
A must-do excursion if you’re in the area is to hire a bangka (P700) near Callao Cave and head 15 minutes upriver to watch the exodus of tens of thousands of bats as they pour out of the caves for a flight over the Pinacanauan River at dusk. If it's raining, the bats won't come out. You can rent kayaks (P200 per hour) from Callao Caves Resort, across the Pinacanauan River from Callao Cave.