Bull sharks are considered to be the most dangerous sharks in the world because they are aggressive, common and often live near heavily-populated areas like tropical shorelines. They often travel inland, swimming up rivers and tributaries. In fact, they've been found thousands of miles up the Amazon River, heading towards Lake Nicaragua. Along with great whites and tiger sharks, they are one of the three sharks most likely to attack humans. Their name comes from their short snout and their tendency to head-butt their victims.