Fun fact: It’s the only marathon in the world that’s technically run on water.

Of the millions of female runners across the United States, only six of them are members of the Marathon Grand Slam Club. And it’s no wonder, considering the intense requirements for membership. In order to be considered, one must complete a marathon distance or longer on each of the seven continents—and on the Arctic Ocean.

While running a marathon in Europe or Australia isn’t too difficult, competing in Antarctica and on the Arctic Ocean proves to be a bit more challenging­­—and frigid. But while 25 U.S. women are recognized as having run a marathon on all seven continents, only six of them can claim membership in the Marathon Grand Slam Club.

So what gives? For starters, there’s only one marathon on the Arctic Ocean: the FWD North Pole Marathon (and it’s not cheap). Entry, which includes a flight from Norway to the North Pole, costs almost $19,000. That, coupled with the cost of the 25+ items on the list of recommended running gear, brings the cost to well over $20,000.

But if you do have the funds to cover it—and don’t mind bone-chilling temperatures—the North Pole Marathon can be a once-in-a-lifetime experience. According to the race website, it’s “the only certified marathon that is run entirely ‘on’ water, the frozen water of the Arctic Ocean.” It’s also recognized as the Northernmost Marathon on Earth.

First run in 2002, the North Pole Marathon was started by Richard Donovan, who ran the marathon distance on his own and became the first person to run a marathon on both the North and South Pole. Since then, the race has grown modestly in popularity. The 2018 race had a total of 60 competitors with the champion, Argyrios Papathanaspoulos, traveling from Greece to win in 4:34. The temperature was -27.4 °F at the start line.

Neither the cold temperatures nor the cost have dissuaded determined runners over the years. The race has welcomed three blind athletes and one wheelchair athlete, who completed the race distance on the aircraft runway. The most senior of North Pole Marathon participants, Michel Ribet, ran in 2016 at the age of 78.

If you’re interested in adding your name to the prestigious list of North Pole marathoners, you’re in luck. Registration for the 2019 race, to be held on April 9, is now open. And there’s no better time to start training than in the dead of winter. Keep in mind though, as much as you may feel like you’re running “in the arctic tundra” this winter, the chilliest of winters is no match for a warm spring day at the North Pole.