A 175ft (53m) hill figure of a white horse discoloured by algae and lichen is being scrubbed by a team of volunteer abseilers.
The Westbury White Horse in Wiltshire, described as a "bit of a grey mare" is being restored in time for the Queen's 90th birthday celebrations.
Up to 20 volunteers have spent the weekend power cleaning the horse, which is carved into a very steep slope.
The English Heritage monument was steam cleaned in 2012.
The cleaning project was organised by the Westbury Rotary Club. Once clean, the hillfigure is due to be given a new coat of white paint.
Under the supervision of an expert climber, two volunteers at a time are lowered down the face of the horse to blast dirt and algae off its surface.
Organiser Steve Carrington said weather conditions over the weekend had been ideal "to get the white horse white again".
"The last thing we want is really bright sunlight because it's really hard on the eyes - it's a bit like snow blindness - so actually slightly overcast with a bit of wind is ideal," he said.
"It was looking a bit like a grey mare and the town council was very keen the white horse was white, as it should be, in time for the Queen's Birthday."
Westbury's horse is said to be the oldest in Wiltshire. It was restored in 1778, but many believe it is far older than that.
It is thought to have been originally carved in 878 AD to commemorate King Alfred's victory over the Danes at the Battle of Ethandune.