This story is part of a special National Geographic News series on global water issues.

Between June and August each year, Hindu pilgrims complete the Amarnath Yatra, an arduous trek to a holy cave high up in the Himalayan glaciers near the border with Pakistan, in Indian-controlled Kashmir. In the past five years, the number of people making the trip has nearly doubled. (See a map of the region.)

In 2011, more than 650,000 people visited the cave. The increased human traffic is threatening the environment in this fragile ecosystem, a major source of water for the Indus River. Scientists are now grappling with how to protect the headwaters of the Indus, while giving reverence to religion, culture, and politics.

Hindu holy man Swami Rama Krishna's bright orange robes stand out like a flame against the throngs of Hindu pilgrims who've come from across the globe to go on the Amarnath Yatra. Undeniable anticipation registers on Swami Krishna's otherwise tranquil face.

Now in the third and final day of his 30-mile journey, he will soon see what Hindus consider to be one of the most holy places on Earth. The pilgrims believe that over 5,000 years ago Lord Shiva, one of the religion's most revered deities, revealed the secret of immortality and creation of the universe in an enormous cave high up in these snowcapped peaks. The ultimate goal of their journey is to go inside the sacred cave to see an ice stalagmite or ice lingam they think is the mark of Lord Shiva.

While this is a journey of a lifetime for many of the worshipers, it's an annual voyage for Swami Krishna. He's been making this passage from his home in New Delhi for more than two decades and says it's his faith that drives him.

"Lord Shiva is my everything," says Krishna, pausing for a moment to chant "Bum Bum Bhulee"—or hail Shiva—as pilgrims clamber by. "I don't want to know the history of the Himalaya or the difficulty of the route. I will go. I want to see him."

It's not hard to see the pilgrims' commitment to faith. Virander Singh, whose legs are shriveled from polio, hobbles to the cave on crutches. At times his frail body hovers dangerously close to the edge of the trail, which drops off hundreds of feet.

"In the name of God, I keep going, despite my burden," he says, fixing his eyes in the direction of the cave, a vertical opening that spans over 100 feet in the mountain face. Last year, over a hundred pilgrims died during the perilous trek to the divine site.

From Crutches to Helicopters

While some pilgrims like Singh and Swami Krishna still choose to take traditional modes of transportation, tens of thousands of others are finding less strenuous approaches.

Besides people who are hoisted up the mountain in carriers lifted by men and thousands of horses, there are now some 300 helicopter flights whizzing wealthy pilgrims to the cave each day. The booming Indian economy and subsidized helicopter flights, which cost about $200 round-trip, make flying to the cave possible for pilgrims who don't want to spend days trekking to the hallowed spot.

Swami Krishna says the deafening sound of helicopters roaring through the Himalaya is very different from how the yatra, or pilgrimage, was just a few years ago.

"I've been coming since very few people were coming," he says, looking nostalgically at the megacity of yellow and blue tents temporarily constructed at the mouth of the cave. "There was no road. There was no track. There were no tents."

With lax restrictions on pilgrims, their numbers continue to rise. And as the number of people increases, so do trash and contamination.

The snowcapped mountains along the trail are now black with the pollution generated from hundreds of thousands of people. To reach the cave, pilgrims walk through piles of garbage, water bottles, gas cylinders, human feces, and occasional horse carcasses. The increased traffic worries Shakil Romshoo, a professor of science at Kashmir University.

"A glacier is subzero, but thousands of people emit radiation at 37 degrees Celsius, whether it is yatri [pilgrims] or anybody," he says. "Start a helicopter and there is a big radiation and temperatures rise. So, definitely that encourages the melting of snow and glacier resources in the region."

Protecting a Vital Water Supply

More than a billion people depend on the water generated from the thousands of glaciers around the Himalaya, which feed the Indus and other major rivers of South Asia. The Indus alone, traveling from the western Himalaya through Pakistan to the Arabian Sea, supports about 90 percent of Pakistan’s agriculture. Scientists are concerned because a number of glaciers in the area are rapidly receding due to climate change.

Romshoo thinks the government needs to intervene by drastically limiting the number of pilgrims and tourists to the region to the carrying capacity of the ecosystem they're in.

David Molden, the director general of the Katmandu-based International Center for Integrated Mountain Development (ICIMOD), agrees.

"Governments have a key role in setting the policy framework and regulations," says Molden. "There needs to be a policy at a local and national level to monitor the flow of tourists in these fragile ecosystems while providing sustainable ecotourism and building the local economy."

Narinder Nath Vohra, the governor of Jammu and Kashmir, says the government has already taken action by no longer allowing the helicopters to land on the glacier next to the cave. Flights are now landing a few miles from the holy site on the banks of a glacier-fed river. The helicopter services, which started in 2003, are a lucrative business. Weather permitting, flight services run every day during the yatra.

Dr. Romshoo believes the service is disastrous for the environment and should be stopped. But many Hindu groups and businessmen say the yatra should be lengthened and the number of helicopter flights increased.

Ayush Bajaj, who runs a night shelter for pilgrims, dreams of one day seeing a paved road with streetlamps leading to the cave.

Some pilgrims like Manula Thaker, who traveled to the yatra from the United Kingdom with her family, don’t believe the increased traffic or helicopter flights are having an impact on the breathtaking landscape they travel across.

"Because of the helicopter, media is saying that the ice stalagmite is melting very quickly," she says. "I would not worry that way because the whole global environment is changing."

But Swami Rami Krishna is not so confident. "The environment has deteriorated," he says as he tries to find a place to wash his face amidst the piles of human feces in one of the glacier-fed streams running alongside the cave. "Actually, the man is the enemy of human society. Man has damaged the human society worse than any other animal, any other plants of the world," he says.