From the 2012 Concurs de Castellsa human tower-building competition in Tarragona, Spain, watch this video by photographer David Oliete. David also took photos:

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A long tradition in the region, castells began at the end of the 18th century. The sport has rules, techniques, and team responsibilities to guard safety and success. The pinya or base is made of a few hundred people that can catch anyone who falls, and the tower itself has a variety of different formations. The top three levels are the pom de dalt, made up of children in helmets. 

While the video above doesn’t show some of the more harrowing challenges, this video by Mike Randolph in 2010 captures why safety and teamwork are so important:

via DesignBoom.