Cueva de Yurac Machay, the "white cave", takes its name from the pale volcanic tuff around it. Inside is a monolith that recalls the ritual stonework at Chavín de Huántar, making the cave an interesting side trip for travellers already focused on the archaeology of the valley.
There is another cave nearby, Cachi Machay. Go with local advice on access and conditions, since this is not as straightforward as visiting the main Chavín site or museum.