As well as being the largest mountain range on the planet, the Himalayas is also one of the youngest. Just seventy million years ago, a very short time in geological terms, the Himalayas did not exist. As the Indo-Australian tectonic plate collided with the Eurasian plate at the rate of about 15 centimetres a year, the ocean floor in between began to rise up to form the mountain range. This means that much of the rock out of which these towering peaks are made was formed at the bottom of an ocean, only to be lifted up thousands of metres into the air. The evidence for this extraordinary journey is not difficult to find. If you look closely at any piece of Himalayan limestone, you will see it has a chalky, granular structure. What you are looking at are the remains of sea creatures. Given a relatively short timescale and a bit of pressure, these biological remains are quickly converted into solid rock. Limestone can also be formed by the direct precipitation of calcium carbonate from water, although the biological sedimentary form is more abundant. We know that the Himalayan limestone is predominantly biological because we have found fossils at the top of Mount Everest. There is perhaps no better example of the endless recycling of Earth’s resources that has been going on since its formation almost five billion years ago.