What is Minecraft and why are millions addicted to it?

Millions of children across the world spend their days digging holes in a virtual landscape called Minecraft. It's an independent game with the most basic of graphics, there's no storyline and you won't see it advertised on TV or posters put up in your local train station.

So how has it managed to shift 54 million copies, with hundreds of new converts each day since its release in 2009, becoming the third most popular video game of all time behind Tetris but ahead of Super Mario Bros?