Around the globe, more and more people are moving into cities. By 2050, the United Nations expects that nearly 70 percent of the world’s population will live in cities—up from 55 percent today (UN.org, May 16, 2018). Densely populated cities put increasing stress on aquifers, naturally occurring underground water reservoirs. As droughts continue and aquifers empty, the structure of the clay and sand in the aquifers changes and begins to collapse.