What are urban explorers?

In a world aglow with electric lights and jam-packed with skyscrapers and cities, are there still uncharted territories left to explore? Sure -- the buildings and tunnels that no one uses anymore. How do urban explorers find adventure in concrete jungles?

There are entire modern abandoned cities dotting the planet, begging to be investigated. Within and underneath functioning cities, sections… » Full Story on HowStuffWorks