The Tesseract is an ancient Asgardian artifact of unimaginable power. It was kept in Odin's artifact chamber but came to Earth by accident. It was kept in a viking church in Norway and kept under the Tree of Life. In World War II Johann Schmidt invaded the church and stole the Tesseract. He used the Tesseract to build extremely powerful weapons that disintegrated humans on touch (the weapons also severely damaged normal objects). Johann also used the Tesseract to make large bombs that could destroy capital cities. Johann held the Tesseract before he sent the missiles and it disintegrated (or maybe teleported to another dimension) him on touch. The cube fell in to the Arctic ocean and was found by S.H.I.E.L.D.'s creator Howard Stark some time later. The Tesseract and Captain America were found in the ocean. The Tesseract was kept under guard by S.H.I.E.L.D. to make sure that it didn't fall in to the wrong hands. In 2011 Nick Fury called in Erik Selvig and asked him whether he would help S.H.I.E.L.D. with making the Tesseract an unlimited power source.