Tracing the human stories generated by the things which skippers loaded in their vessels to ensure the ships' stability once their paying cargoes had been unloaded, and the people, plants and wildlife that travelled with them.
Generations of sailors have had to understand how to shift ballast to keep their vessels stable.
Wrongly loaded ballast means disaster.
But the cargoes of ballast have had some remarkable side effects at ports around the world.
There are the homes built from 'foreign' materials or the islands created by debris dropped in estuaries which now house foreign plant species.
There are the pollution problems caused by thousands of species being moved around the world in a single day in water ballast on tankers.
And there are the remarkable stories of how skippers loaded human ballast and charged people to live in the holds on hazardous journeys.