Why do we represent journeys as lines on maps? Maps do not show the world as it really is - they show us how the mapmakers see it, and they are the product of centuries of trading, exploring and conquering. The lines recording individual journeys are even more revealing: they first appeared in the blank seas of Renaissance maps, and signalled a momentous transformation in Europeans' worldview.In Tracks on the Ocean, Sara Caputo tells the story of journey lines - from Ferdinand Magellan and Captain Cook's routes across the South Seas, to the disorientating power of digital technology to reshape how we see our world. These personal trails are key to understanding the origins of surveillance, and they mark humanity's impact on the planet, whether in the legacy of violence plotted by imperial endeavours or in the clouds of exhaust fumes left by ocean liners.