A rare set of perfectly preserved 'ghost' footprints have been found at a US Air Force base.
The ancient archaeological impressions date back an estimated 12,000 years, and were uncovered at the Test and Training Range in Utah's West Desert following a detailed survey of the ground.
The set of 88 shapes have since been dubbed 'ghost footprints', as they only become visible to the human eye after rain has fallen and moisture causes them to change to a darker colour.
Researchers first discovered them by accident while en route to an established archaeological site at the Hill Air Force Base in Utah's Great Salt Lake Desert.
After originally finding a small number at the location, a more thorough excavation turned up the full range of footprints appearing to have been left by a group of ancient humans of various ages and sizes — potentially a multi-generational family.
Daron Duke from Far Western Anthropological Research Group was part of the team that found them, and described the discovery as an important "insight" into the "daily life of a family group thousands of years ago".
He added: "Based on excavations of several prints, we’ve found evidence of adults with children from about 5 to 12 years of age that were leaving bare footprints"
"People appear to have been walking in shallow water, the sand rapidly infilling their print behind them -- much as you might experience on a beach -- but under the sand was a layer of mud that kept the print intact after infilling."
Mr Duke says researchers will now look to preserve the footprints and protect them against extreme weather conditions that could damage them, such as high winds and erosion from rain.
Infill has also been collected from prints to source organic materials for a carbon dating process.
Members of the Far Western Anthropological Research Group will also speak to Native American tribes to find their perspectives on the prints.
Commenting on the find, Anya Kitterman, Cultural Resource Manager at the Department of Defense, said the prints represented a "once-in-a-lifetime discovery" and added: "We found so much more than we bargained for".
