When we think about the end of the world, it’s usually zombies running amok, alien invasions and nuclear winter. But American engineer Eric Drexler presented an alternate world-ending hypothesis. In his book ‘Engines of Creation’, he talked about ‘Gray Goo’ - self-replicating nanobots that consume all matter on earth while simultaneously building more of themselves. It’s like if your Roomba got a little too ambitious and decided to clean the entire planet, hoovering up all biological matter in its path.

In theory, these molecular-level robots could actually be helpful to humans: they could solve the world’s energy crisis, cure diseases and create any product at a low price. Of course, all of those upsides come with the slightly concerning risk of turning the planet into a lifeless mass teeming with nanomachines.