Dead dinosaurs.

Also in Gubbio, or just outside actually, is the most famous geological outcropping that no one's ever heard of.  In 1977, Walter Alvarez and his team discovered a layer of iridium, between two layers of rock, in an exposed cliff.  Iridium is plentiful in meteorites but exceedingly rare on earth.  He proposed that a giant meteorite strike occurred some 60 million years ago, which was so massive it sent dirt and ash into the atmosphere, which effectively blocked the sunlight virtually everywhere in the world.  The plants died without sunlight, the animals that ate the plants died, the animals that ate the animals that ate the plants died.  The dinosaurs had ruled the planet, and now were gone.  The scientific community initially snorted in derision at this idea, but the scientific community can be pretty sure of itself, until it isn't.  It's now accepted science, and here's the rocky cliff that provided the evidence.  Somewhere in all these rock layers is the KT Boundary, with strata from the Cretaceous era and from the Tertiary, with a thin layer of iridium in between.