It's hard to believe, but millions of years ago the Alps sat at the bottom of a vast temperate sea, leaving behind millions of fossils pressed into the rock – including ammonites, trilobites and even the footprints of prehistoric birds. Spread across 18 sites collectively known as the Réserve Géologique de Haute-Provence, they offer a fascinating window into the Alps' ancient past – but you'll need a car (and the official map, available at tourist offices) to find them.
Interpretive panels explain each of the sites, most of which lie around Barles (north) and Barrême (south). One of the easiest to reach is an impressive limestone slab with some 500 ammonites, about 3km north of Digne on the road to Barles.
You'll need a full day to cover the whole route – especially if you want to combine it with a visit to the Musée Promenade.