You can knock down a stone wall with a sledgehammer. A few excavators and some wrecking balls would take care of a five-story building. But when you need to bring down something really big, say a 20-story skyscraper, you have to haul out the big gear. Explosive demolition - blowing up stuff -- is actually the safest and most efficient way to demolish larger structures. When a building is surrounded by other buildings, you can implode the building-to make it collapse in on itself. That way the surrounding buildings won't be torn down, too.
So, how do you make a building collapse in on itself? The basic idea is actually pretty simple. If you remove the support structure of a building at a certain point, the section of the building above that point will fall on the structure below. If this upper section is heavy enough, it will crush the lower part. The explosives are just the trigger-gravity actually brings the building down.
Professionals known as "demolition blasters" load explosives onto support columns at several different levels of the building. That way the building structure falls down on itself at multiple points. The main challenge is controlling which way it falls.
Ideally, a blasting crew will be able to tumble the building over on one side, into a parking lot or other open area. It's a lot like chopping down a tree. To topple the building to the north, the blasters detonate explosives on the north side of the building first. You would chop into a tree from the north side to tip it that way, too. Blasters may also secure steel cables to support columns in the building and pull the support columns in the direction the blasters want the building to fall.
When other buildings are right next door, the blasters have to stage a true implosion. They must make the building collapse straight down into its own footprint (the total area at the base of the building).
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