As millions of years stretch into tens of millions, the transformations come more slowly. Rare earth minerals, leached from discarded mobile phones and other electronic devices, may begin to form secondary mineral crystals. Glass from windshields and shop windows will devitrify, darkening just as obsidian does after long burial. By now, the entire city is compressed to a layer perhaps only a few metres thick in the strata. All that is left of Shanghai Tower is a geological anomaly studded with the fossil outlines of chopsticks, chairs, sim cards, and hair clips.