Alexander von Humboldt (1769-1859) is the great lost scientist: more things are named after him than anyone else. There are towns, rivers, mountain ranges, the ocean current that runs along the South American coast, there's a penguin, a giant squid - even the Mare Humboldtianum on the moon. Taking us on a fantastic voyage in his footsteps, Andrea Wulf shows why his life and ideas remain so important today.