In the controlled demolition industry, building implosion is the strategic placing of explosive material and timing of its detonation so that a structure collapses on itself in a matter of seconds, minimizing the physical damage to its immediate surroundings. Despite its terminology, building implosion also includes the controlled demolition of other structures, such as bridges, smokestacks, towers, and tunnels. Building implosion, which reduces to seconds a process which could take months or years to achieve by other methods, typically occurs in urban areas and often involves large landmark structures. The actual use of the term "implosion" to refer to the destruction of a building is a misnomer. This had been stated of the destruction of 1515 Tower in West Palm Beach, Florida. "What happens is, you use explosive materials in critical structural connections to allow gravity to bring it down." Terminology The term building implosion can be misleading to laymen: The technique is not a true implosion phenomenon. A true implosion usually involves a difference between internal (lower) and external (higher) pressure, or inward and outward forces, that is so large that the structure collapses inward into itself. In contrast, building implosion techniques do not rely on the difference between internal and external pressure to collapse a structure. Instead, the goal is to induce a progressive collapse by weakening or removing critical supports; therefore, the building can no longer withstand gravity loads and will fail under its own weight. Numerous small explosives, strategically placed within the structure, are used to catalyze the collapse. Nitroglycerin, dynamite, or other explosives are used to shatter reinforced concrete supports. Linear shaped charges are used to sever steel supports. These explosives are progressively detonated on supports throughout the structure. Then, explosives on the lower floors initiate the controlled collapse. A simple structure like a chimney can be prepared for demolition in less than a day. Larger or more complex structures can take up to six months of preparation to remove internal walls and wrap columns with fabric and fencing before firing the explosives. Historical overview As part of the demolition industry, the history of building implosion is tied to the development of explosives technology. One of the earliest documented attempts at building implosion was the 1773 razing of Holy Trinity Cathedral in Waterford, Ireland with of gunpowder, a huge amount of explosives at the time. The use of low velocity explosive produced a deafening explosion