Scientists say artificial gas tracers pumped into water at the surface of the Greenland ice sheet revealed a hidden network of rivers flowing beneath it.

Tracking the flow of water underneath giant ice sheets may give clues about the ways in which Greenland and Antarctica's enormous ice masses are reacting to our changing climate and how they might contribute to sea-level rise, the researchers said.

"Before we started this project five years ago, nobody really knew if and how water flowed at the bed of ice sheets," Jemma Wadham of the University of Bristol in Britain, one of the study's authors, said.

A team working nearly 40 miles inland pumped a fluorescent liquid and a traceable gas into water at the surface, while another team waited at the ice-sheet margins on the Greenland coast, waiting for the tracers to appear.

'It's quite a tense time -- you're sort of sampling in the dark because you don't know for sure where and when the tracers will come out," Wadham said.

The speed with which the tracers appeared suggested they had been carried in fast-flowing rivers beneath the ice, she said.

The study results will be fed into computer forecasts of ice-sheet responses to our warming climate, the researchers said.

This could have important consequences for understanding the overall stability of the ice sheets, a release from Britain's Natural Environment Research Council, which funded the study, said.