What happened in Flint seemed unthinkable in modern America.Water in the Michigan city was discovered to be contaminated by high levels of lead, a known neurotoxin that is especially dangerous to children and pregnant women. As the crisis unfolded — leaving children covered in rashes or at risk of severe neurological impairment , or even death— many of us asked ourselves how it could have happened right under our noses.Though the crisis started in early 2014, it didn't make national news until December 2015, when Flint Mayor Karen Weaver declared a state of emergency in the city over the proliferation of lead in the water supply.Within weeks, Michigan Gov. Rick Snyder deployed the National Guard, and President Obama declared the situation a federal emergency. But the damage had already been done. Within months, the country was asking how the crisis could even have been allowed to happen in the first place.Two years after the crisis started, Obama plans to visit Flint for the first time (thanks in part to one young girl's efforts ). The president will hear firsthand about the situation and what still remains to be done.Here's what you need to know before his visit.In April 2014, the city of Flint, MI, changed its water source from the city of Detroit’s Water and Sewerage Department to the nearby Flint River in an effort to save money. The switch was intended to be a temporary measure while the city built a pipeline to the Karegnondi Water Authority, a project expected to save the region about $200 million over 25 years, according to NPR While the city issued several alerts over the detection of E. coli bacteria and possible carcinogenic chemicals in the water, the fatal flaw was in the water treatment. The Flint River has a history of heavy pollution , and the water that came from the river was more corrosive than the water that had been coming from Detroit. Officials did not treat the water against corrosion, and as it passed through the old lead and iron pipes of Flint’s infrastructure, lead began leaching into the water.In February 2015, independent testing by researchers from Virginia Tech found lead levels in one home between 200 parts per billion (ppb) and 13,200 ppb, a number so outrageously high that investigators thought they had made a mistake until retesting confirmed the findings.The World Health Organization’s maximum allowable lead level is 10 ppb, and the Environmental Protection Agency’s actionable level is 15 ppb.Not enough, critics of the government's handling of the situation say. Authorities are accused of dismissing residents' concerns, falsifying safety tests, and having an overall incompetent response to public safety. Within a month of the switch , residents started complaining about the color, smell, and taste of the water. But the complaints were dismissed, with authorities assuring the public that the water was safe to drink — even after the Virginia Tech study found extremely high levels of lead. An investigation into the crisis, led by Michigan Attorney General Bill Schuette, alleged in April that water treatment authorities falsified tests to show lower levels of contamination than were actually taking place, leaving residents vulnerable.