meadows", as it featured abundant wild grasses, as well as the desert spring waters needed by westward travelers. History Nomadic Paleo-Indians traveled to the Las Vegas area 10,000 years ago, leaving behind petroglyphs. Ancient Puebloan and Paiute tribes followed at least 2,000 years ago. A young Mexican scout named Rafael Rivera is credited as the first non-Native American to encounter the valley, in 1829. Trader Antonio Armijo led a 60-man party along the Spanish Trail to Los Angeles, California in 1829. In 1844, John C. Frémont arrived, and his writings helped lure pioneers to the area. Downtown Las Vegas's Fremont Street is named after him. Eleven years later, members of the Church of Jesus Christ of Latter-day Saints chose Las Vegas as the site to build a fort halfway between Salt Lake City and Los Angeles, where they would travel to gather supplies. The fort was abandoned several years afterward. The remainder of this Old Mormon Fort can still be seen at the intersection of Las Vegas Boulevard and Washington Avenue. Las Vegas was founded as a city in 1905, when of land adjacent to the Union Pacific Railroad tracks were auctioned in what would become the downtown area. In 1911, Las Vegas was incorporated as a city. 1931 was a pivotal year for Las Vegas. At that time, Nevada legalized casino gambling and reduced residency requirements for divorce to six weeks. This year also witnessed the beginning of construction on nearby Hoover Dam. The influx of construction workers and their families helped Las Vegas avoid economic calamity during the Great Depression. The construction work was completed in 1935. In late 1941, Las Vegas Army Airfield was established. Renamed Nellis Air Force Base in 1950, it is now home to the United States Air Force Thunderbirds aerobatic team. Following World War II, lavishly decorated hotels, gambling casinos, and big-name entertainment became synonymous with Las Vegas. In 1951, nuclear weapons testing began at the Nevada Test Site, northwest of Las Vegas. During this time, the city was nicknamed the "Atomic City". Residents and visitors were able to witness the mushroom clouds (and were exposed to the fallout) until 1963 when the Partial Nuclear Test Ban Treaty required that nuclear tests be moved underground. In 1955, the Moulin Rouge Hotel opened and became the first racially integrated casino-hotel in Las Vegas. The iconic "Welcome to Las Vegas" sign, which has never been