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A couple thousand years ago, the Greek historian Plutarch posed an interesting question. Ships were made of wood at the time, and wood rots when exposed to water. So sailors were forever replacing bits of wood on a ship. What Plutarch wanted to know was, as the wood was replaced, at what point was it the same ship, and at what point was it a different ship?

It’s an intriguing question. If the sailors replaced a single piece of wood – one of the masts, say – almost everyone would agree that it was still the same ship. But what if the sailors replaced 51% of the wood? Would it still be the same ship then? What if that 51% was replaced over a long period of time? What if they replaced 49% of the wood and 10 years later they replaced the remaining 2%: would the 49% have been “grandfathered” as part of the original ship? If there was only a single piece of wood from the original ship left, but they built an entirely new ship around it, isn’t it still the same vessel as before? After all, a car enthusiast might buy a beat-up classic car and replace almost every single part as part of the restoration process. Is it not the same car?

Americans might recognize Plutarch’s thought experiment as “George Washington’s ax”.

In 1800, American author Parson Weems released his most famous work, a hilariously inaccurate biography of the nation’s first president called The Life of Washington. The book contained a completely fabricated tale about a young Washington being given an ax by his father; little George then went about chopping down anything he could find, including his father’s favorite cherry tree. When confronted, Washington reportedly said that he “could not tell a lie” and admitted his misdeeds.

The likelihood that this actually happened is near zero: Weems held Washington as a god, and it wouldn’t surprise me in the least to hear that Weems claimed Washington single-handedly defeated the British, found the lost city of Atlantis, and dug the Grand Canyon all by himself, too.

Still, Weems’ book was required reading in American schools for much of the 19th century, and teachers used the ax to illustrate Plutarch’s riddle: if you replaced the handle of George Washington’s ax, was it the same tool? What if you replaced the ax head a few years later? What if Washington’s family used the ax for a 100 years, and both the head and the handle had been repeatedly replaced… was it still the same ax?

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One thing that is true is that various parts of ships were reclaimed over the centuries. Hemp ropes that outlived their usefulness were tweezed apart into individual strands called oakum, a type of caulking often used on ships. People who lived near ports often bought cheap reclaimed wood and used it to build stuff, or bought it to resell to other people who used it to build stuff. Like this barn in Buckinghamshire:

The barn was built by Thomas Russell in 1624, using timbers from a ship named… Mayflower. In the 1920s, English historian J. Rendel Harris confidently declared that the wood came from the Mayflower, the ship that transported the Pilgrims from Plymouth to New England. For decades, the barn was a tourist attraction, especially with Americans (it’s privately-owned and now closed to the public). Coincidentally, the barn is a stone’s throw from the grave of William Penn, founder and namesake of Pennsylvania.

Thing is, though, there were at least 37 other ships called the Mayflower at the time. So chances are slim that the wood is from the Mayflower. But it’s always a possibility. It is said that the wood was from a shipbreaker’s yard in Rotherhithe, which most certainly did exist, and likely was the final resting place of the Mayflower. But we’ll never really know for sure.

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It’s thought that the Mediterranean was the birthplace of the sea trade, between Phoenicians, Greeks, Egyptians and others. And one of the first things those intrepid sailors would have learned about was weight.

If you put too much “stuff” on a ship, it rides low in the water, which makes it prone to sinking. Such ships are less maneuverable in the water, and if a few waves crash over the sides and into the cargo hold, the ship can lose buoyancy and sink. On the other hand, if you don’t put enough “stuff” on a ship, the opposite happens: it tends to ride high in the water, making it prone to tip over and sink.

Of course, this was easy to deal with on the voyage out: just make sure the ship isn’t too full, and she won’t ride low and sink. But it was a big problem on the way home, when cargo holds were often empty. So sailors started adding weight, or ballast, to empty ships to make them more stable in the water. Ballast could be anything at all, so long as it was heavy. They could have used lead balls or iron bars. But the cheapest option, by far, were plain old rocks. So a ship would arrive in port and empty its hold of goods. Most traders preferred to fill the hold with goods to take home to sell; if not, the crew would replace the missing weight with same amount of rocks.

For centuries, rocks from all over the world ended up on opposite sides of the globe. A little bit of Egypt ended up in Greece. A little bit of Fiji ended up in France. Little bits of India ended in up in England. That this happened isn’t in the least bit remarkable. In fact, marine archaeologists look for ship-sized piles of ballast rock when looking for shipwrecks.

Then World War II happened. Supply ships would leave New York with cargoes full of food and medical supplies for our English allies. Ships that safely arrived in Bristol would empty their cargo. But they needed ballast for the voyage home, and that’s when a unique situation presented itself: the Germans had bombed the hell out of Bristol, and the city had piles and piles of rubble everywhere. So the rubble was used as ballast, and when the ships arrived in New York they dumped that ballast in the exact same place. Eventually, the ballast built up so much that it formed a new piece of land:

That little triangular bit near Bellevue Hospital on the east side of FDR Drive is made up almost entirely of former bits of Bristol. While it’s not unusual to find bits of volcanic rock from Hawaii at a port in, say, Turkey, it is unusual that so much ballast from one specific place was dumped in another specific place.

I’ll let English YouTube star Tom Scott take it from here: