Satellites, coral reefs, ancient Roman fishponds and sinking cities help us understand how humans are changing sea level.

The alien invaders arrived quietly. Only one man noticed as they drifted down from the sky. Donald Barber was an astronomer at Norman Lockyer Observatory. Its telescopes sat on a grassy hill surrounded by farmland on the south coast of England. Barber was using the telescopes to measure the light from far-off stars. He captured the starlight in photographs produced on glass plates coated with chemicals, like the film in an old camera. It was only after Barber developed those photographic plates in the summer of 1937 that the first signs of tiny alien invaders emerged. The astronomer’s disc...