Scientists are increasingly concerned about the potential for permafrost to thaw in a warming world.

This permanently frozen layer of earth contains huge amounts of methane that could exacerbate climate change if the gas leaches into the atmosphere.

To help study the issue, the US Army Corp of Engineers has built a tunnel into permafrost near Fairbanks, Alaska.

It allows researchers to travel back in time to see how permafrost impacted the Earth over the past 40,000 years.

BBC environment correspondent Matt McGrath travelled with them.