Four RAF personnel are taking on a daring challenge this year - to climb the 38 floors, 180 metres and more than 1,000 steps of the Gherkin in London.
It is arguably the most instantly-recognised tower in the city.
The aim of those taking part is to scale the building 50 times, raising money for the RAF Benevolent Fund.
The challenge, which started 10 years ago, and has seen more than 6,000 people take part.
Some have termed it "the stairway to hell."
Senior Aircraftman Anna Duckett, RAF Northolt, said:
"They've been supporting the RAF, service personnel, veterans, their families for 100 years so it's a nice marker for us to say thank you."
Echoing the sentiment, was Senior Aircraftman Sam Lake, RAF Northolt:
"Obviously it's close to the Royal Air Force's heart, the benevolent fund has supported the RAF since 1919 - so the year after its inception really.
"It's nice to give something back to a charity that supports and does so much for the members of the air force."
Regional organiser, from the RAF Benevolent Fund, Charlotte Barmby was full of praise for the participants:
"It's a really tough challenge.
"You've got no windows, you've got no air-conditioning, and you're constantly spiralling up the building, so good on them for giving it a go."
"We have all sorts of people doing all sorts of different things to raise money for the RAF benevolent fund, but this is one of those events that really stood out.
"It was completely unique, at a really famous London landmark to celebrate the 100 years of the Royal Air Force - it was just the perfect combination to do something different and raise money for a good cause so I was really excited to work with them on it."
It's a formidable test of stamina, and once complete - the 360-degree view across London awaits them at the top.