For centuries boats were ballasted with sand, gravel, stone, or rubble to give them stability, and when they no longer needed the extra weight, it was dumped. The result was that huge quantities of ballast were shipped to new places and new continents.

In Ballast the archaeologist Mats Burström charts how ship ballast helped to shape the world we live in. Ballast was often reused, sometimes in surprising ways. With the ballast went animal and plant life of all sorts, inadvertently spread to places where they are now so well established that they are thought of as native species. And it was not unknown for ancient artefacts to be found in the ballast too, turning up in the most unexpected places.

This is the first comprehensive account of ship ballast, so long overlooked, and now finally recognized for its diverse and exciting history.