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.
