Climbing season on Mt. McKinley, in Denali National Park and Preserve, attracts hundreds of climbers from around the world each spring and summer.

They arrive at the Talkeetna airport knowing that for a few weeks they'll be at the mercy of something much greater than themselves.

It starts with the weather. Conditions have to be just right before any of the air taxis will take off.

On this day, pilots get word from high above that conditions are mostly clear -- it's a go.

The journey begins.

While flying in, passengers often cannot see their goal, but they know it's there, somewhere behind the clouds.

It's a destination most people can only dream of from a distance.

Most of the Lower 48 refers to North America’s tallest peak as Mt. McKinley. In Alaska, they call it Denali.

To say the climb begins at base camp would be misleading.

It takes months if not years of preparation and for some the trek is much more than an escape from the ordinary.

“I haven't been on a trip quite like this,” said Alexandra Levin, a 30 year-old Harvard-educated New Yorker who works on Wall Street and knows a thing or two about risk and reward.

And she knows now how important it is to seize an opportunity.

“Getting diagnosed with MS changed my perspective and made me think about wanting to push myself more, I guess, and living in the moment and doing what I can while I can do it,” she said while packing her gear at base camp on a recent June morning.

Multiple Sclerosis might someday rob her of the ability to even climb a staircase.

But months of arduous training dragging weights along a New York beach, and injections of disease-modifying medicine, mean she has a pretty good shot at unfurling her National MS Society banner at the summit.

“Hopefully it's a good luck thing,” she said.

The sense of purpose that drives Denali climbers cannot be overstated.

It seems like every other person at base camp is doing the trip not only for themselves, but for others back home.

They say that support is essential to their success, but there's a whole lot more fueling the voyage.

“We eat a lot of high-calorie snacks. I’ve got Oreos in there, I have cheese, I have goldfish crackers, which I really like,” said Levin as she rummaged through her pack.