With roughly 60 percent of Americans now living in urban zones of more than 200,000 residents, finding a quick escape from big-city stress has become a lot harder. But some large metropolitan areas are lucky enough to have undeveloped tracts of nature so large, they can offer a taste of wilderness within their city limits. Here are six of our favorite spots.

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Rock Creek Park, Washington, D.C.

A long swath of green cutting through the northwestern quadrant of the nation's capital, this park created in 1890 offers a slew of amenities, from a planetarium and an amphitheater to a golf course and tennis courts. At more than 2,000 acres, it's the largest urban natural park administered by the National Park Service, and it offers plenty of breathing space. Thirty-two miles of hiking trails meander through wetlands, meadows and a deciduous forest of oak, hickory, sycamore and maple. Only a few miles from Capitol Hill, the park nonetheless teems with animal life, including white-tailed deer, foxes, beavers, neotropical migratory birds and the country's largest concentration of raccoons. 

Griffith Park, Los Angeles, California

With more than 4,210 acres, this municipal park and wilderness area offers the 3.8 million residents of Los Angeles — and at least one resident mountain lion — an appropriately sprawling space to get away from it all. An observatory, the Greek Theater, the Hollywood sign and other famous attractions dot the perimeter of the park. But its location in the sometimes rugged Santa Monica Mountains, at elevations ranging from 384 to 1,625 feet, has ensured that a huge portion remains as it was when Native American villages occupied its lower slopes. During hikes through chaparral-covered terrain and deep canyons, you'll encounter few people but plenty of other California natives, such as local species of oak, walnut, lilac, mountain mahogany and sumac. 

Bayou Sauvage National Wildlife Refuge, New Orleans, Louisiana

Within the city limits of New Orleans, less than 30 minutes by car from raucous French Quarter revelry, lies a quiet world — part land, part water — where the American alligator is the most common predator. Bayou Sauvage, one of the largest urban national wildlife refuges, covers 24,000 acres and includes some of some of the last remaining marsh adjacent to Lake Pontchartrain. Hurricane Katrina exacted a toll here, especially on mature coastal hardwood trees, and full reforestation will take years. But the refuge remains a beautiful, tranquil place where visitors can canoe, hike and fish, or check out the 340-plus species of birds and the peak population of 25,000 waterfowl who call it home.