HEMET -- Thump. The room shook.

Thump. The ground trembled.

Thump. Middle-schoolers, all cooler than cool when they entered the darkened theater-in-the round, suddenly had "Where's Mommy?" looks on their faces as a 12-foot-tall mastodon stomped across the movie screen.

The giant creature emerged from the ancient forest and not only shook up the teens in the theater but also the animated saber-toothed cats, giant sloths and other prehistoric creatures on the 270-degree screen.

At the new Western Center for Archaeology and Paleontology, the floor of the state-of-the-art theater shakes at appropriate moments adding to the sense of realism for visitors as they view a film about the creatures that roamed this region thousands of years ago. Another film focuses on the massive construction project that built Southern California's largest drinking-water reservoir.

Located next to the east dam of Diamond Valley Lake, the 33,000-square-foot museum opened in October and showcases the prehistoric treasures -- including giant sloths, massive mastodons, and pottery fragments dating back 9,000 years -- unearthed in the 1990s during construction of the lake.

Where are the dinosaurs?

That most-often-asked question of docents is answered in Archaeology Hall on one of the first stops on the museum tour, at an exhibit called "The Big Dam Hole."

The display shows how work crews dug out rocks and dirt down to a depth of nearly 270 feet during reservoir construction. As they dug, artifacts, bones, tools and fossils dating back nearly 230,000 years were unearthed.

But no dinosaurs.

During the Jurassic period -- also known as the Age of the Dinosaurs -- about 200 million years ago, the west coast of North America was entirely under water, explained Chris Hansen, the museum's education director, to a group of students on a field trip from Lakeland Village Middle School near Lake Elsinore.

"That's why there were no dinosaurs here," Hansen said.

Still, there are plenty of Ice Age animals to study at the center, as well as lessons in more modern history.

Another exhibit in the hall shows parts of the re-created Nicholson Homestead, a typical Southern Californian home of the 1880s. A prehistoric archaeology alcove exhibits how the native Luiseno and Cahuilla Indians lived in the valley -- now filled with 260 billion gallons of water -- thousands of years ago.

A real Big Dig

In 1987, with the population of the region expected to double within 50 years, Metropolitan Water District began a study to determine a location for a much-needed reservoir. Four years later, they decided on the Domenigoni and Diamond Valley site -- 12,000 acres of farmland, dairies and poultry ranches nestled between several hills just north of Temecula.

After clearing environmental and property-acquisition hurdles, ground was broken for the project in 1995 and work began on the east dam -- one of the project's three dams -- in June 1996.

Work had barely begun when the first of the ancient treasures was uncovered.

Just 30 feet below the surface, paleontologists found the partial skeleton of a long-horned bison, a species thought to have died out 25,000 to 30,000 years ago.

At 55 feet, the remains of a giant mastodon were uncovered.

As the project continued, more discoveries were made. Scientists carefully extracted the fossils of dozens of ancient animals from the ground even as giant earthmovers thundered nearby.

Some animals, like the dire wolf, black bear, North American lion and long-horned bison, had not previously been found in Riverside County or the Inland Empire.

Eventually, more than 6,000 fossils were recovered from more than 1,000 sites within the reservoir project.

Tools, bones, mortars, clay pipes and pottery were also uncovered at the site revealing more than 8,000 years of continuous human occupation.

In addition, evidence of ponderosa pine and manzanita suggested the region was more forested and wooded than it is today.

Valley of the Mastodons

During the Diamond Valley dig, the area became known, unofficially, as the Valley of the Mastodons.

More than 500 mastodon fossils were uncovered at 111 different sites, including the partial remains of a 10-foot-tall, 13,000-year-old specimen, dubbed "Max, the largest mastodon ever uncovered in the western United States.

Inside the museum's main exhibit hall, "Max" stands next to "Xena," a 12-foot-tall, 10,000-year-old mammoth, and not far from the remains of "Little Stevie," a mastodon that lived in the valley 50,000 years ago.

"One unique feature here is that in most museums you see the animals re-created," said Roxanne Roundtree, the Western Center's marketing and events manager. "Here, you see the bones we actually found at this site."

Attendance has grown steadily since the museum opened in October, Roundtree said.

School groups from Riverside to Temecula and points in between visit almost every day and take a two-hour tour guided by docents.

Those not part of an organized group can rent headsets for a nominal charge and take a self-guided tour.

In addition, the facility has scientific laboratories used by students from nearby Mt. San Jacinto College and conference facilities used by both private and public entities.

She'll be there for you

Some kids may want to be Rachel and others aspire to be Joey, but hardly any youngster wants to grow up to be Ross, the paleontologist on the popular and oft-rerun TV sitcom "Friends."

However, while many of her classmates from Lakeland Village Middle School were in one corner of the museum's 10,000-square-foot main exhibit hall making fossils out of clay, across the room 14-year-old Brenna Dugan pored over research materials.

"I really want to be a paleontologist," she said.

Nearby, two boys giggled.

"Shut up," she said to them. "It's true."

Brenna said she has had discussions with her mother and read many books on the subject.

"I've never been interested in anything else but this," she said.

More giggles.

"Shut up," she said again.

With that, the boys wandered off in the direction of the many hands-on exhibits inside the hall and Brenna returned to her reading.

Meanwhile, not far away and inside the museum's theater, another group of students sat wide-eyed as the giant mastodon rumbled across the movie screen.

In the darkened theater, with the floor shaking below, one youngster turned to another and said, simply, "Cool."

Contact staff writer John Hunneman at (951) 676-4315, Ext. 2603 or hunneman@californian.com.

VERY OLD BONES

What: The Western Center for Archaeology & Paleontology

Where: 2345 Searl Parkway in Hemet. From Southwest County, take Winchester Road north. Turn right on Domenigoni Parkway and turn right again on Searl Parkway.

When: 10 a.m. to 5 p.m., Tuesday through Sunday. Closed Mondays.

Cost: $8 for adults, $6.50 for seniors and college students, $6 for youths. Military and children 4 years and younger are admitted free.

Info: www.westerncentermuseum.org. Call (951) 791-0033.