Rainbow Bridge is a sandstone arch that is nearly 300 feet tall, making it one of the world's largest natural bridges. When researchers stuck seismic sensors on and near it, they measured eight different types of vibrations. Most of these small movements were natural. But energy carried through the ground from distant, manmade events also made the bridge resonate. These included waves from Lake Powell, an artificial reservoir, and an induced earthquake in Oklahoma (these are usually caused when people get rid of wastewater from oil and gas drilling by injecting it into the earth).