The Devil's Throat is a slot canyon cut into the soft clay and salt of the Catarpe valley, north of San Pedro past the Pukará de Quitor. The walls close to a couple of metres apart in places and rise thirty metres above your head, folded and striped where the sediment was tilted by the mountains going up behind it. The floor is dry sand, so you walk or ride straight up the middle.
This is the best bicycle outing from San Pedro and one of the few things here you can do without booking anything. It is about 8 kilometres each way on dirt from the plaza, flat most of the way, with a rental bike from any of the shops on Caracoles costing a few thousand pesos for the day. Horseback tours cover the same ground. Take more water than you think and go in the morning, because there is no shade in the valley and none on the way back.
Higher up the same valley is the Túnel de Catarpe, so the two make a natural loop.