Seven marathons, in seven continents, over seven consecutive days.
No Singaporean had done this before, but cousins Ong Tze Boon, 47, and Ong Yu-Phing, 46, managed to pull off this remarkable feat last month. And they plan to run another marathon, in the North Pole, in April.
The Ong cousins were among 15 participants of the annual World Marathon Challenge, which is in its second run this year. They ran a total of 295km across seven continents - starting with Antarctica, through South and North America, and on to Europe, Africa, Asia and Australia. They had to cope with temperatures ranging from minus 15 deg C to about 25 deg C, different terrains and different time zones.
Mr Ong Tze Boon, the son of late former president Ong Teng Cheong, told The Straits Times: "It was good to start the challenge in the Antarctica, where it's all daylight in the summer. After that, and with all the flights, we just lost reference of what's day or night."
The cousins are set to join the ranks of over 130 members of the 7 Continents Marathon Club, which recognises runners who finish a marathon distance on each of the seven continents, regardless of the time taken. Only three other Singaporeans - Dr William Tan in 2007, Ms Gloria Lau in 2012 and Ms Yvonne Chee in 2013 - have done this, but not in seven days in a row.
Through the World Marathon Challenge and North Pole Marathon, the cousins hope to raise funds for five charities that support mental health: Alzheimer's Disease Association, Caregivers Alliance Limited, Samaritans of Singapore, Singapore Association for Mental Health and We Care Community Services.
The World Marathon Challenge is the only event in which people can run seven marathons in seven continents over seven days.
It was started last year by Global Running Adventures, which is owned by Irish marathon runner Richard Donovan. Dubbed the "Marathon Man", he holds the world record for running seven marathons on seven continents. He first set the record in 2009, at the age of 42, when he completed the feat in under six days. Three years later, he beat this with a timing of four days, 22 hours and three minutes.
Runners run in the seven locations in this order: Union Glacier, Antarctica; Punta Arenas, Chile; Miami, US; Madrid, Spain; Marrakech, Morocco; Dubai, United Arab Emirates; and Sydney, Australia.
Daniel Cartica and Becca Pizzi from the United States won the men's and women's titles respectively this year.
Before the World Marathon Challenge - they ran their last marathon in Sydney, Australia, on Jan 30 - they did the Marathon des Sables (French for "marathon in the sands") last April, a 250km race in the Sahara Desert which Discovery Channel named the "toughest foot race on earth". Through this, they raised $85,000 for Club Rainbow, which helps chronically ill children.
For Mr Ong Tze Boon, group executive chairman of environmental design solutions firm Ong & Ong, that ultramarathon was his first marathon. He began running regularly in mid-2012 and, after a year, could run 10km a day. But he was not keen to join the crowded local marathons.
He decided on an endurance run and encouraged his cousin to join him in Marathons des Sables. He said: "We'd done desert and heat, so the next alternative would be cold and ice. Then I learnt of the World Marathon Challenge and decided to go for the 'package deal'.
"No Singaporean has ever done it in seven days. And if we don't do it now, then when?"
Both cousins said the toughest leg was in Africa. It was their fifth marathon in the challenge, and they had run three in the previous two days. Mr Ong Tze Boon also suffered a right knee strain and had to walk for the final two laps, while his cousin had an adductor strain.
They are resting their bodies and will train more intensively next month for the North Pole Marathon, which they must run in temperatures of about minus 45 deg C.
Completing it would give them entry into the Marathon Grand Slam Club - for those who complete a marathon on each of the seven continents and the Arctic Ocean - and boost their fundraising drive. They have raised about $140,000 so far.
It is fitting that mental endurance is needed for these efforts to help the mentally ill, said Mr Ong Yu- Phing, an IT director at Ong & Ong.
"There was a 92km stretch that we ran over 24 hours in last year's race in the Sahara desert, and we were supporting each other then. If he was down, I would cheer him up, and vice versa. Having the support made that stretch more doable.
"It is also important for the mentally ill to have a social support system. Having social support can really help people do what they otherwise may not have been able to do."