For millions of years, the amount of water on Earth has remained the same. The amount of freshwater, however, has become scarce thanks to a population explosion and human-influenced climate change. Droughts, floods, rising sea levels, and saltwater contamination of freshwater resources have forced us to confront the challenges facing humanity today. See how science has responded, with solutions such as seawater desalination on an industrial scale, sewage water purification via microfiltration, and programs to address floating plastic debris.