Digimon >> Data Squad

From the introduction:

Ten years ago, a crisis swept the world as string of missing person cases involving people of all ages and nationalities; their common link being how sudden and inexplicable each one was. It was only after a young couple witnessed the disappearance of their son that the cause was determined; portals to a strange new world that seemed to spring into being for unknown reason. Worse still, the phenomenon went both ways, as inhabitants of this strange world would also fall into ours. These creatures calling themselves digimon proved to be highly sensitive to human emotions; causing them to run rampant until returned to their digi-eggs, or chancing upon another digi-gate.