The wreck of a Roman ship was discovered 25 years ago, and nobody could explain why a large lead pipe was smashed through the hull. Now we know it's just another example of Roman engineering prowess. Specifically, it's a fish tank.
It's easily taken for granted, but the idea of transporting foods over long distances is a relatively modern concept. After all, any meat is going to rot without refrigeration, and it's not easy to place the necessary cooling equipment on a moving vessel. Of course, you can just use nature's way of keeping meat fresh by transporting live animals, but that presents its own set of technical challenges.
That's where the ancient Roman fish tank comes into play. Historians had long assumed that any fish caught in ancient times were eaten nearby. But this ancient shipwreck reveals an ingenious method for keeping fish alive indefinitely, and the Romans could have taken their fish all across the Mediterranean Sea.
The lead pipe through the hull would have allowed water to be pumped in and out of the boat, but you don't drill holes in your boat unless you've got a very good reason to. This particular ship was small - only about 54 feet long -
making it unlikely that the sailors made it to drain bilge water or fight fires as they might have done on a larger boat. However, containers recovered from the wreck have the remains of processed fish like sardines inside them, indicating it was part of the fish trade.
That's where the fish tank hypothesis comes from. Writing for Nature, Jo Marchant explains how the tank would have worked:
The researchers calculate that a ship the size of the Grado wreck could have held a tank containing around 4 cubic metres of water. This could have housed 200 kilograms of live fish, such as sea bass or sea bream. To keep the fish alive with a constant oxygen supply, the water in the tank would need to be replaced once every half an hour. The researchers estimate that the piston pump could have supported a flow of 252 litres per minute, allowing the water to be replaced in just 16 minutes.
Now, to be fair, it's possible that the researchers have dismissed the other possibilities a little too quickly, and there certainly is no evidence of the tank itself. But this finding does fit with primary sources about ancient Roman trade. Pliny the Elder mentions that live parrotfish were shipped all the way from the Black Sea to Naples, where they were then placed back in the sea in the hopes of establishing a new population.
What's particularly intriguing is how small the boat is - if it really did carry a fish tank, then that's a good indication that the practice was commonplace, and even small-time merchants made use of the technology. It's just more evidence that perhaps the ancient world wasn't quite so ancient after all.