One important line of evidence lies inside the world's glacial ice. AdvertisementScientists like to describe glacial ice as a kind of annual record book. Hardening glacial ice can also trap volcanic ash, which lets us know when an ancient eruption must've taken place. In mid-December, scientists announced they had an ice core of huge historical importance that was removed from the Tibetan Plateau. All the corpses on record were buried in permafrost (i.e., frozen soil) when paleontologists recovered them, not glacial ice.