Tuffs have been a part of man's environment for thousands of years and underlie some of Earth's largest cities. There are 41 large industrial cities in 24 nations (including two megacities) that are underlain or partly underlain by tuffs. Rome, one of the world's most famous cities, has a history tied closely to the tuff deposits upon which the ancient city was built. Tuffs are products of explosive volcanic eruptions and are composed of volcanic ash and pumice particles that are bonded by natural cements or are naturally welded; they make excellent building materials and have been proposed as a medium for industrial and nuclear waste storage. Tuff deposits are often hundreds of meters thick and can cover hundreds to thousands of square kilometers. The most common tuffs used for building material are ignimbrites (pyroclastic flow deposits), in which the pumice and glass shards have been sintered by heat immediately after deposition or have been bonded by natural cements precipitated from fluids percolating through the deposits. When used for building stone, ignimbrite is sawn or broken away from a quarry face along natural cooling joints and then fashioned into blocks by hand or with power saws. These blocks, with enough strength for multiple-story buildings, stone walls, and other structures, are resistant to weathering, are lightweight, and have good insulating properties—better than most other natural building stones.