From Alaska to the Everglades, from Maine to Maui, the United States comprises more than 2 billion acres, and one out of every three of those acres belongs to you.

As a nation that never had a monarchy, the United States has no royal preserves set aside for the king and his cronies. Since the birth of the republic, rather, any land not privately claimed has been held in trust for the American people. Today, the nation's public lands range from the tiny rings of green around traffic circles in Washington, D.C., to the Wrangell-St. Elias Wilderness in Alaska, a 9 million-acre swath of tundra that is the biggest single public parcel.

The public lands include the highest and lowest points in the nation, the country's tallest waterfall, the largest ski resorts, the headwaters of most major U.S. rivers, and such national treasures as Yellowstone National Park, the Grand Canyon and the Blue Ridge Parkway. They are key sources of mineral and energy wealth, and centers of biological diversity. A biosurvey of a single city park in Danbury, Conn., in 2001 found 2,519 distinct living species (including 300 types of beetles).

For residents of Nevada, where the federal government owns 83 percent of all land — not to mention Alaska (68 percent), Utah (64 percent) and Idaho (61 percent) — public-land ownership is a basic fact of daily life and a key source of political dispute. But most Americans have considerably less involvement with the public lands they own.

That's why the last Saturday in September each year has been designated National Public Lands Day, when tens of thousands of people volunteer a day of their time for cleanup, paint-up and fix-up on city, state and national lands.

The 12th annual public-lands day will take place Saturday. Anybody who wants to help can volunteer at 800-865-8337 or www.npld.com.

As part of the celebration, entrance fees to national parks will be free from midnight to midnight.

"It's a wonderful way to give something back to these areas that add so much to American life," says Robb Hampton, program director of the national day for its sponsor, the private National Environmental Education & Training Foundation.

Hampton says that some 750 public sites will have jobs for volunteers Saturday, ranging from upgrading hiking trails to repairing bridges and removing graffiti.

Federal-land managers say the need for volunteers is particularly strong this fall, because thousands of park and forest employees have been sent to the Gulf Coast for emergency response after Hurricane Katrina.

The United States has held large areas of public land as long as there has been a United States. The original 13 states ceded some 240 million acres to the new national government at its birth (the federal government, in turn, agreed to assume the states' Revolutionary War debts). The Louisiana Purchase, the Alaska Purchase and others added more than 1 billion acres in public holdings.

Under the Homestead Act of 1862 — championed by Abraham Lincoln "so that every poor man may have a home" — the government gave about 80 million acres, for a small fee, to war veterans and Western settlers. The last of the homestead acts was not repealed until 1986.

Today, federal land comes in the form of parks, forests, lakeshores, battlefields, monuments, military installations and the National Mall along Constitution Avenue in Washington, D.C. There is a National Historic and Scenic Trails System, a National Landscape Conservation System and a National Wild and Scenic Rivers system. All 50 states maintain state parks and preserves, with a national total greater than 3,300.

The most stringent controls on public land are found in the National Wilderness Preservation System, plots of land where roads, structures, machines, weapons, pets and vehicles (even bicycles) are banned.

When the Wilderness Act was passed in 1964, the system began with 9 million acres set aside in 13 states. Today there are 106 million acres of wilderness areas, in 44 states; Rhode Island, Connecticut, Delaware, Maryland, Iowa and Kansas have no federally protected wilderness.

The public lands produce constant political friction. The most heated current disputes surround the Bush administration's determination to open more public land to mineral and energy development.

But the burning 19th-century debate as to whether the federal government should own land at all seems to be settled. Under the Federal Lands Policy and Management Act of 1976, Congress expressly promised "to retain federal lands in public ownership." There is no discernible movement in contemporary politics to reverse that policy.