Unlike many Washington state attractions, the word "majestic" does not apply to the slopes and swales of the Mima Mounds Natural Area Preserve. But walk amid the mounds, and you will be struck by how their subtle uniformity adds up to an otherworldly power.

The swath of grassy humps known as the Mima Mounds Natural Area Preserve spins a mystery as yet without an ending. And with any good story, context is critical.

©Chris Joseph Taylor / The Seattle Times
Summertime visitors who stroll along the Mima Mounds' interpretive trail will find themselves amid humps covered in lichen and moss, along with other vegetation, butterflies and bees. The color palette of the landscape, which is south of Olympia, changes with the seasons.

A single 6-foot-tall-by-30-foot-wide mound in a grassland prairie is but a mound. But acres upon acres of mounds, sharing similar shape and size and packed in clumps like eggs in a carton?

What are they? Why are they here? No one knows.

The hummocks have starred in their own geological whodunit - or whatdunit - since Capt. Charles Wilkes, leader of the U.S. Exploring Expedition that charted the Northwest, happened upon them in the mid-1800s.

Wilkes thought they might be Native American burial sites, but found only earth inside. Geologists and others have since developed several dozen hypotheses, including glacial freeze-and-thaw cycles, erosion, interplay between wind and vegetation, an earthquake or two, a tsunami or perhaps a volcanic eruption.

One popular theory credits generations of grubbing pocket gophers with creating the mounds while building underground dens. But there is nothing close to consensus.

Unlike many Washington attractions, the word "majestic" does not apply to the slopes and swales and open space here. But walk amid the mounds and you will be struck by how subtle uniformity adds up to an otherworldly power.

You'll brush past salal, huckleberries, Indian plum and humps carpeted by pale lichen and white moss. The open landscape changes with the seasons. Spring sprouts dainty blue Camas lilies. Butterflies and bees flit about during summer's warmth. Fall rain changes the color template back to monotone green.

The mystery of the mounds is the hook that pulls people off Interstate 5 and west through the nearby burg of Littlerock, Thurston County (named after a rock in the original postmaster's yard), to take a look. But the geologic debate can obscure the fact that the mounds populate a habitat strikingly out of place in the western half of the state, one that is steadily shrinking.

The 637-acre preserve, which is home to open grassland and oak woodlands, is sandwiched between the freeway and the Black Hills and Capitol State Forest to the west. It used to be part of a 180,000-acre prairie system that stretched from McChord Air Force Base down to Chehalis and Oakville in Grays Harbor County. About 3 percent of it remains today - what's left is only in pockets.

Long before white settlers moved in, Native Americans regularly burned the land to maintain the prairie so it would continue to bear Camas lilies and other plants valuable to their lives. Widespread burning is now impractical because homes dot the land.

Douglas firs have nudged their way onto the prairie, along with the doggedly invasive Scotch broom. As a result, some of the plants unique to these prairie systems are considered threatened, endangered or at risk of decline.

The Department of Natural Resources (DNR), the Nature Conservancy and scores of volunteers work to maintain and restore the area. On one recent morning, rain-soaked grass slathered the boots and pantlegs of DNR natural-areas manager Birdie Davenport and volunteer Cliff Snyder as they walked around the mounds.

"If it wasn't for all the clearing our volunteers do, this whole area would be full of Scotch broom," Davenport says. "We have done a few controlled burns, but that's hard to do around here. So we use other methods, mostly."

Davenport and Snyder stopped before a few 10-inch-tall native, but threatened, golden paintbrush plants. The plants look like Popsicles, but their soft yellow beams against the pale-green grassland represent hope. The golden paintbrush, once abundant along the West Coast, has dwindled along with the ecosystem that supports it.

The plot is part of several transplanting and seeding studies, carried out by state and federal agencies and the Nature Conservancy.

Snyder, a master gardener who helps with stewardship of the preserve and works other prairie areas nearby, is one of several volunteers who spend hours upon hours each week ripping out invasive plants and occasionally reintroducing natives.

The prairie was created when glaciers retreated 15,000 years ago and left fast-drying gravelly soil behind, but that doesn't explain the mounds.

The preserve came about through the early work of scientists, notably Vic Scheffer, a former University of Washington professor, photographer, naturalist and author. He first visited the mounds in the early 1940s, after one of his students, Walter Dalquest, persuaded him to check them out. Scheffer's initial opinion: "Wow."

Davenport says Scheffer and other scientists, along with the Nature Conservancy, led a drive to protect the mounds. By 1966 it had achieved National Natural Landmark status and soon after became a protected natural preserve.

Scheffer and Dalquest maintained that as glaciers retreated, vegetation took root on the thin soil. Gophers - a lot of them - moved in, too. The creatures dug chambers but encountered the thick layer of glacial debris beneath the soil. They moved soil and pebbles upward, forging mounds.

Some other theories:

- Gravel, stones and soil that washed upon a melting ice-age glacier may have collected in pits called "suncups." As the ice melted, the sediment pits settled in mounds.

- Earthquakes sent shock waves through the soil and moved the earth into mounds where the peak waves intersected.

- The mounds are deposits from sediment-rich floods from a glacially dammed lake. Some believe the mounds come from sediments deposited where water worked its way around vegetation.

On and on it goes.

Complicating agreement is that Mima-like mounds dot other places across the world in places with different ecosystems. Even the origin of the word "Mima" (MY-ma), which was coined here, seems fuzzy.

One mound, inside a DNR gravel pit a few miles from the preserve, has been sliced open. The bottom two-thirds consists of the glacial outwash, pale and full of pebbles. The top third is made up of black silty earth that could be fine charcoal from repeated burning, many scientists believe.

Scientists don't wonder just about the mounds - they wonder about the evolution of the entire ecosystem. Linda Storm, an ethnobotanist, spent years trying to understand the relationship between Native American practices and the land.

She waxes poetic about the prairie because she has come to understand and appreciate how the ecosystem moves to its own rhythm.

"The otherworldly aspect of it is part of what connects me to the prairie," she says. "The spirit of the place that calls me through the plants and Native American oral histories and realizing that these beautiful grasslands had a long-term relationship with humans. Here is beauty and magic in understanding the diversity of the place, from the plants to the birds, butterflies and solitary bees pollinating."