Save. Jupiterimages/Photos.com/Getty Images. It's easy to mistake lichens as the cause of tree or branch death because they tend to grow on sick trees. In fact, the reason they turn up is because a sick tree has a thin canopy and lets in the sun that makes lichens grow.ave. Jupiterimages/Photos.com/Getty Images. It's easy to mistake lichens as the cause of tree or branch death because they tend to grow on sick trees. In fact, the reason they turn up is because a sick tree has a thin canopy and lets in the sun that makes lichens grow.
4 Hoover Dam. Hoover Dam is one of the world's great engineering marvels. This massive structure, completed in 1935, crosses the Colorado River, linking Arizona and Nevada. It is 726 feet high and 1,244 feet long. Lake Mead, held back by the Hoover Dam, is the largest artificial lake in the United States. It is 110 miles long, and holds the equivalent of two years of flow of the Colorado River.
Named one of the Top 10 Construction Achievements of the 20th Century, Hoover Dam continues to draw crowds 80 years after its creation. Attracting more than a million visitors a year, Hoover Dam is located in Black Canyon, just minutes outside of Las Vegas. On April 20, 1931, construction began on the Hoover Dam, the largest of its kind at the time. Despite the remote location and harsh working conditions, it was completed on March 1, 1936 in less than five years -- two years ahead of schedule -- and well under budget.