When volcanoes erupt, they may emit a variety of materials, such as lava, gases, and pyroclastic rocks, which may vary greatly in size from sand-like particles to sizable boulders. If accumulations of the sand-sized grains become lithified over time, the material that results is often referred to as volcanic sandstone. Because of its relation to eruptions, this type of sandstone is most prevalent in areas that have historically been associated with heavy volcanic activity, such as Naples, which stands in the shadow of Mt. Vesuvius. Due to the unique qualities of volcanic sandstone, one of the most complex and expansive underground systems in the world can be found beneath this Italian city. Though the carving of this amazing underworld was begun thousands of years ago, large numbers of Naples inhabitants have utilized the system of chambers and aqueducts as recently as the 1940s, when they served as emergency air raid shelters during World War II.