Just outside Scottsdale, Arizona, sits one of the most expensive piles of rocks in the world.
Nearly invisible from most angles, amid granite boulders more than a billion years old, is a three-bedroom, 2.5-bathroom home. The 9-acre site recently hit the market at $4.2 million. (Click here or on any photo for a slideshow.)
It's not just the walls of rock that make this home unique.
In order to fully form and seal the house without disrupting the native arrangement of the boulders, architect Charles Johnson had to design additional stucco walls to enclose the home and custom-build windows to mold them into the cracks and crevices between boulders. The boulders sometimes jut into rooms, and other times the rooms flow around them.
Boulder House, as it's known, was commissioned in 1974 by Sunnie and Bill Empie, native Arizonans looking to return to the dry heat from the wet Pacific Northwest, according to Curbed.
They stumbled across a newspaper ad looking for a buyer for a beautiful pile of rocks, listing agent Preston Westmoreland told Yahoo Homes.
When they saw the 9-acre site, they fell in love with it and immediately wanted to build on it.
"The original plan was to build the home next to the boulders," Rick Cibik, a member of the Fort McDowell Yavapai Nation that now owns the property, told Good Morning Arizona. "Charles Johnson wanted to build the home within the boulders."
It took several years to complete construction, and the Empies made it their home in 1980. During construction and their time living there, the couple continually found proof that people had lived there long before the Empies had: Ancient pottery was scattered on the site -- and 20 petroglyphs celebrating the womanly body had been carved into the boulders.
The site was probably used more than 1,000 years ago as a fertility site or birthing place, Cibik said. Artifacts found in the fireplace in a bedroom date back about that far, he said.
Sunnie Empie wrote of one particularly dramatic discovery:
"It was mid-March, only a few months after we had moved into our Boulder House, when we first noticed the dazzling six-inch-wide dagger of sunlight stretching slowly across the polished concrete floor. The late afternoon sunlight entered through a narrow split between twenty-foot-high boulders that make up the west wall of our home. Each day the shaft of light on the floor lengthened, and then it climbed upward over a low granite boulder, moving slowly in a searching way toward a thirty-inch spiral carved in the stone. At 5:25 p.m., the light finally reached the petroglyph. Stone nodules at the edge of the spiral glistened like diamonds in the sunlight to signal the completion of the sundagger's journey. Then the sun vanished behind the boulder wall.
"We were astonished to witness the sun interact with the petroglyph, but even more staggering, this was happening on March 21 -- the first day of spring.
"Some people see stones as silent sentinels. Some say wistfully, 'If only the rocks could talk.' A millennium ago, tapping sounds echoed among the cluster of boulders as an artisan wielded a hammerstone and created symbols in the granite to honor the life-giving force of Nature.
"When the sun penetrates a symbol or enters the cave, it is remarkable that a hole or entry has been created to receive the sun, a feature that appears to honor the procreative power of the sun. The sun enters the space, the womb of the earth."
The light reaching the mark indicates the spring and autumnal equinox, making it an ancient calendar, Cibik said.
Discoveries like these led to the Boulder House's inclusion on the National Register of Historic Places; it's now known as the Empie Petroglyph Site.
Westmoreland, the listing agent, reports that interest has been keen since this pile of rocks hit the market.
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• 10 incredible homes built into rocks (52 photos)
• Couple lives under a rock, literally (with photos, video)
A local TV segment about Boulder House from a few years ago: