Nowadays, almost all battle tanks are similar in design, but it wasn't always like that. Here are 21 tanks from the ages that never made it to the battlefield.
Carvings from 700BC illustrate some of the first battle-ready, wheeled transportation devices that could pass as 'tanks'. Technically, they're more like battering rams with archers on the top, but that's because explosives hadn't been invented yet.
Some have speculated that these early inventions were the inspiration for the wooden horse of Troy. Foot-soldiers would be hidden beneath a deck, pushing it along on either four or six wheels. Soldiers would crouch on the platform, ready to spring into action once the battering ram had breached the enemy's walls.
For the next few hundred years, this design would evolve, but couldn't have been practical in open warfare. It was only with the invention of the catapult, and when crossbows were fixed to these chariots that things started to change.
It is around 113AD, when images of these appear on Trajan's Column in Rome, that historians see the idea of a tank for the first time. The Chinese would introduce gunpowder to the world in 904AD, but it would take another four centuries to make it to Eurasia.
Some of the earliest illustrations of tanks with turrets are from the 1400s, but many contained design flaws that would make them impossible to manoeuvre. Others would be too heavy or unsuitable for troops to operate when the guns would fire.
Needless to say, even with legendary visionaries like Leonardo da Vinci, the realisation of the tank as we would describe it today was thanks to the internal combustion engine, armour plating and the continuous 'caterpillar' track.
The video below by YouTuber, Lindybeige, is a fascinating look at many of the machines dreamed up by inventors over the centuries. There are 21 tank prototypes in this display, with scale models made of each so we get a decent idea of what they might have looked like.
It's a startling reminder of how quickly humanity has changed over the millennia. Imagine where we'll be in another 1,000 years...