Denver was made territorial capital in 1867. Gold and silver strikes in the 1870s–80s brought prosperity, and the city became the capital of bonanza kings such as H. A. W. Tabor. In the late 1890s, Denver's development as a metropolis began. After World War II, during which military bases brought development, Denver experienced rapid growth; this, combined with the city's high elevation, led to environmental problems, and by the late 1970s Denver had one of the worst U.S. smog problems.
Denver boomed again in the late 1970s as a center of oil shale exploration, and many new office buildings were erected in anticipation of further growth. When oil prices fell in the 1980s, the city was hard hit economically, and population loss to its booming suburbs accelerated. By the 1990s, however, international and government-related business and tourism had brought another boom. The city added a light-rail transit system in 1994, and a huge new international airport opened to the northeast in 1995. By the late 1990s the Denver region, after concerted efforts to improve air quality, had significantly reduced the level of air pollution.