“The cloud” is a real place. The pictures you post on Instagram, the happy birthday wishes you leave on Facebook pages, and the TV shows you stream on Netflix aren’t living in a nebulous ball of condensation in the sky. They live on a massive series of servers – all connected together in rows and towers in giant warehouses.

Few people have ventured into these data centres. But in the Swedish capital Stockholm, I went inside these information labyrinths, and discovered that they’re not just housing data. All the heat they give off is helping to warm homes in the city of over 900,000 people. How does it work? And could it create a new business model for the tech industry worldwide?

Inside the labyrinth

Walking through a data centre you notice a few things. The air is cool and dry. And there’s not a dust bunny to be found. The rows of server towers are covered in thousands of blinking lights and there are rarely any people. Everywhere you look across the ceiling and underneath the removable floor tiles are masses of cables running in every direction.