An American explorer is trekking across Antarctica alone with only one pair of underwear. He just made it to the South Pole.

Courtesy of Colin O'Brady

• 33-year-old American explorer Colin O'Brady is attempting to become the first person to cross Antarctica alone and unaided.
• He just reached the South Pole, 40 days into his trek.
• A British man, Louis Rudd, is also trekking alone across the southern continent and nearing the South Pole.
• If either man finishes the journey, it would be a world first. 

It's literally all downhill from here for American explorer Colin O'Brady.

The 33-year-old is trying to become the first person to cross Antarctica on skis, alone and unaided. He just hit the South Pole - the route's halfway mark and point of highest elevation. O'Brady expects he'll pick up speed now as he heads down-slope and away from the pole. His final destination is the Ronne Ice Shelf.

O'Brady posted an Instagram photo of himself at the bottom tip of the world on Wednesday, 40 days into his trek. "Today has quite honestly been one of the best days of my entire life," he wrote in the post. "It was whiteout conditions approaching the pole as it's been for days." 

The whiteout was so bad the night before O'Brady reached South Pole that he stopped and camped in his tent, just three miles shy of his goal. Once he reached the pole, though, conditions were clear enough for him to snap the selfie.

Just before O'Brady started his journey, he told Business Insider that the first half of the trek would be harder than the second. He took off hauling a 400-pound load, which included just one pair of underwear in order to save weight.

Read More: At the bottom of the world, a 33-year-old is about to trek across Antarctica alone - a journey no one has survived. He's bringing just one pair of underwear.

"All the way to the South Pole, not only do I have a heavy sled, but I'm going uphill the entire time," he said. "It's imperceptibly up and downhill to the naked eye, but you can really feel it when you're dragging that much weight." 

The Impossible First/Colin O'Brady/Business Insider

Now, O'Brady has eaten 40 days' worth of food rations - he's subsisting on a 8,000-calorie-per-day diet of protein bars and freeze-dried meals - and burned through a good chunk of his cooking fuel.

So he's hoping to pick up the pace. 

But O'Brady isn't the only man trying to set a record as the first person to cross the white continent without the help of a re-supply or a kite. Around the same time he started his trek, another explorer, 49-year-old Louis Rudd, set out from the other side of the continent. Rudd is now 12 miles away from the South Pole checkpoint, which means the world first is still in reach for either skier.

The solo, unaided crossing both men are attempting has never been completed before because everyone who's tried has either given up or died.

"People have crossed oceans in sailboats by themselves, people have even rowed boats across the oceans, but this is just one of those things that hasn't quite been accomplished," O'Brady said.

That's part of the reason why O'Brady bulked up before the trip. He spent months strength training, packing about 20 extra pounds of muscle mass onto his normally 165-pound frame.

O'Brady said he hopes his journey inspires others to do things they never thought were possible, too.

The Impossible First/Colin O'Brady

O'Brady on a previous trek at the North Pole.