The next time you're looking at a six-story building, take a moment to consider that 100 million years ago, you could have been staring up at an Argentinosaurus. At 120 feet long and weighing 100 tons, this creature holds the title of the biggest dinosaur ever to walk the planet. While much about this dinosaur remains a mystery, experts do know that it had a long neck, a long tail and a relatively small head. One fossilized vertebra alone measures more than five feet crosswise. This planteater was first discovered in Argentina in 1988.