and as the crowd grew larger, the National Guard started shooting. Four students were killed and nine injured. This event caused disbelief and shock throughout the country and became a staple of anti-Vietnam demonstrations. Environmentalism The 1970s started a mainstream affirmation of the environmental issues early activists from the 1960s, such as Rachel Carson and Murray Bookchin, had warned of. The Apollo 11 mission, which had occurred at the end of the previous decade, had transmitted back concrete images of the Earth as an integrated, life-supporting system and shaped a public willingness to preserve nature. On April 22, 1970, the United States celebrated its first Earth Day, in which over two thousand colleges and universities and roughly ten thousand primary and secondary schools participated. Sexual Revolution The 1960s counterculture movement had rapidly undone many existing social taboos, and divorce, extramarital sex, and homosexuality were increasingly accepted in the Western world. The event of legalized abortion and over-the-counter birth control pills also played a major factor. Western Europe was in some ways more progressive on sexual liberation than the United States, as nudity in film and on TV had been gradually accepted there from the mid-1960s, and many European countries during this time began allowing women to go topless in public places. Nudist culture was also popular during the decade, especially in Germany and Scandinavia. Child erotica found a niche market, but would eventually be banned under child pornography laws in the 1980s to 1990s. The market for adult entertainment in the 1970s was large, and driven in part by the sizable baby boomer population, and the 1972 movie Behind the Green Door, an X-rated feature, became one of the top-grossing films of the year. Playboy Magazine appeared increasingly dull and old-fashioned next to new, more explicit sex-themed magazines such as Penthouse Magazine and Hustler Magazine. By the end of the decade, there was an increasing backlash against libertine sexual attitudes, and the event of the AIDS epidemic helped bring about an end to the Sexual Revolution. Adult movie theaters, which had exploded in numbers during the 1970s and were widely seen as a symptom of urban decay in the US, declined as pornographic movies would largely shift to VHS tapes during the succeeding decade. Crime and urban decay Crime rates in the US had been low from the 1940s until the mid-1960s, but began to escalate after 1965 due to