Just when we thought we knew everything important about the age of sail and its impact on the world, along comes research that exposes our collective myopia when gazing at the past. Ships carried all sorts of things across the oceans, including commodities, disease, and ideas. But as archaeologist and author Mats Burström shows, ships also scattered bits of their homelands—quite literally—around the world.

Today, ships use water as ballast, in the process delivering microscopic organisms to international ports. But once upon a time, mariners relied on solids—bricks, stones, and gravel—to lend their ships stability. In four centuries of sailing, they left millions of tonnes of material around the globe from Canada to India.

In this excerpt from his new book Ballast: Laden with History, which has its North American release this week, Burström reveals the surprising story of ballast and considers its place in archaeology. Is historical ballast an artifact or a natural object?