The Grotte di Valdemino are a karst cave system on the outskirts of Borgio, carved through limestone by the underground Rio Battorezza stream. The tourist route, opened in 1970, covers 800 metres and passes through chambers of striking colour — white, yellow, ochre and deep red — caused by iron and manganese oxides in the rock. Specialists have called it the most colourful show cave in Italy, and the claim is easy to believe when the lights catch the overlapping tones on a single chamber wall.
Several small underground lakes punctuate the route, their surfaces perfectly still and mirror-flat. Ancient bone fragments found in the cave — dated to between 500,000 and 750,000 years ago — point to animal occupation long before any human settled on this coast. Guided tours depart regularly in season and last about 45 minutes; the caves maintain a constant temperature of around 16°C, so a light layer is worth bringing.