Fibre-optic cables usually make us think of ultra-fast internet – or maybe the irritation of new installation works digging up the pavement. But there are now such cables snaking their way through the centre of the planet’s second largest body of ice, the Greenland ice sheet. Fibre-optic technologies are allowing us to monitor the internal structure of glaciers in unprecedented levels of detail. In our new study, we show how fibre-optics are offering extraordinary new insight into how ice sheets evolve – and how the movement of Greenland’s glaciers is far more complicated than previously thought. Ice loss from Greenland has increased sixfold since the 1980s, and the melting ice sheet is now the single biggest contributor to global sea level rise. In order to forecast the ice sheet’s future – including its worrying rates of melting – we need to understand the thermodynamic processes at work within it. That means we need to take its temperature as accurately as we can.