Eight thousand years ago, forests covered more than 23 million square miles, or about 40 percent of Earth's land surface. Today, almost half of those forests have fallen to the ax, the chain saw, the matchstick, or the bulldozer. A map unveiled in March by the Washington-based World Resources Institute not only shows the locations of former forests, but also assesses the condition of today's forests worldwide. Institute researchers developed the map with the help of the World Conservation Monitoring Center, the World Wildlife Fund, and 90 forest experts from a variety of universities, government organizations, and environmental groups. Only one-fifth of the remaining forests are still 'frontier forests', defined as a relatively undisturbed natural forests large enough to support all of their native species. Frontier forests offer a number of benefits: They generate and maintain biodiversity, protect watersheds, prevent flooding and soil erosion, and stabilize climate. Many large areas that have traditionally been classified as forest land don't qualify as 'frontier' because of human influences such as fire suppression and a patchwork of logging. 'There's surprisingly little intact forest left,' says research associate Dirk Bryant, the principal author of the report that accompanies the new map. In the report, Bryant, Daniel Nielsen, and Laura Tangley divide the world into four groups: 76 countries that have lost all of their frontier forest 11 nations that are 'on the edge' 28 countries with 'not much time' and only eight—including Canada, Russia, and Brazil—that still have a 'great opportunity' to keep most of their original forest. The United States is among the nations said to be mining out of time: In the lower 48 states, says Bryant, 'only 1 percent of the forest that was once there as frontier forest qualifies today.' Logging poses the biggest single threat to remaining frontier forests. 'Our results suggest that 70 percent of frontier forests under threat are threatened by logging,' says Bryant. The practice of cutting timber also creates roads that cause erosion and open the forest to hunting, mining, firewood gathering, and land clearing for farms. What can protect frontier forests? The researchers recommend combining preservation with sustainable land use practices such as tourism and selective timber extraction. It's possible to restore frontiers,' says Bryant, 'but the cost and time required to do so would suggest that the smart approach is to husband the remaining frontier forest before it's gone. What is the main idea of the passage?