Reuter / Corbis Zoroastrians pray around a fireplace inside their temple in the village of Chak Chak.

Far removed from Tehran's bustling tin-roofed teashops and Isfahan's verdant pomegranate gardens, the deserts known as Dasht-e Kavir and Dasht-e Lut meet at the city of Yazd, once the heart of the Persian Empire.

Walking across the wind-whipped plains of the forgotten city, a young Iranian woman dressed in colorful floral garbs points out a sand-dusted tower hovering in the distance like a dormant volcano under a relentless sun. "This is where we put tens of thousands of corpses over the years," she explains with a congenial smile.

The funerary tower is part of the ancient burial practice of Zoroastrianism, the world's oldest monotheistic religion. Zoroastrians (known in India as Parsis) regard sky burials, in which the bodies are exposed to natural elements including vultures in open-topped "Towers of Silence," as an ecologically friendly alternative to cremation, consistent with their religion's reverence for the earth. A Zoroastrian priest clad in a long, cotton robe explains: "Death is considered to be the work of Angra Mainyu, the embodiment of all that is evil, whereas the earth and all that is beautiful is considered to be the pure work of God. We must not pollute the earth with our remains."

The priest believes that open burials are a fulfillment of the central tenet of his religion, which is to practice good deeds. With a forlorn expression, he notes that, 3,000 years after the tradition of open burials began, there are not enough Zoroastrians left alive to keep the tower in Yazd open. Instead, today's Zoroastrians who want to observe traditional burial practices must request in their will that their body is sent to a forested suburb in Mumbai, India, where the last Tower of Silence still operates.

In the alabaster prayer room of the Zoroastrian temple in the center of Yazd, a handful of adherents sway to the cadence of ancient Persian prayers recited as a priest feeds sticks of sandalwood and sprinkles of frankincense into a blazing urn. Zoroastrians wear hand-woven wool cords as external symbols of their faith, and almost always pray in front of a fire, which represents purity and sustainability. In Yazd, the holy flame has burned for 1,500 years without ever being extinguished. While Zoroastrianism was once the dominant religion in a swathe of territory spanning from Rome and Greece to India and Russia, the number of adherents has dwindled exponentially over the centuries. Although Yazd is the birthplace of the religion, only 200 of its 433,836 people still practice Zoroastrianism because migration, forced conversions, and centuries of oppression have diminished the population.

Worldwide, there are 190,000 Zoroastrians at most, and perhaps as few as 124,000 by some estimates. Although Zoroastrians are few in number, their faith has influenced Judaism, Christianity and Islam with its teachings of a single deity, a dualistic universe of good versus evil, and a final day of reckoning. The religion professes that humankind is designed to evolve toward perfection, but is complicated by evil forces such as greed, lust and hatred, explains Mehraban Firouzgary, the head priest of the Zoroastrian temple in Tehran. According to Zoroastrians, these evil forces must be challenged proactively by developing a "good mind" that embraces a life of good thoughts, good words and good deeds.

Despite their shrinking population, Zoroastrians remain fiercely divided over whether to recognize interfaith families, let alone accept non-generational Zoroastrians. Tens of thousands fled Persia during the Islamic incursions in the 10th Century and were granted refuge in India under the condition they did not marry outside their faith or proselytize to the Hindu majority. Ramiyar P. Karanjia, principal of a Zoroastrian religious school in Mumbai, India, insists, "Conversion is not part of our religion." Yet, in India, home to the majority of Zoroastrians, the community is declining by about 10% every decennial census, according to a report released by UNESCO. Today, Zoroastrians remain a tight-knit and self-secluded community that strongly encourages marriage within the faith.

According to Parva Namiranian, a Zoroastrian medical student at Tehran University, the community in Iran preserves its identity by learning the Persian poetry of the Shah Nameh and holding religious classes and celebrations. She says Zoroastrians are accepted in Iran because they "represent a proud history" and all Iranians, regardless of religion, enjoy celebrating the Zoroastrian New Year, Nowruz, because it's an excuse to buy clothes and eat sweets. Mehraban Firouzgary, the head priest in the Zoroastrian temple in Tehran, agrees that most Iranians regard the Zoroastrian minority favorably, but he worries about the community's survival. "Zoroastrians have lived in Iran for over 3,000 years," he says, "but there are so few left today."