By Tran Thi Minh Ha with Alice Philipson in Hanoi One summer morning, Le Thi Hong Mai’s home collapsed into a river in Vietnam’s Mekong Delta, where shoreline erosion caused by sand mining and hydropower dams threatens hundreds of thousands of people. Sand — needed to produce concrete — is the world’s second most exploited natural resource after water, and its use has tripled in the last two decades, according to the UN environment programme. […]