When a mountain of trash collapsed at the fetid Reppi dump outside of Addis Ababa on March 13, at least 82 people died. It could've been worse: Hundreds of people live atop Reppi, Ethiopia's biggest waste dump, trying to make a living from salvaging what city residents throw away. Despite well-known dangers, and the best efforts of the government, they've done so for decades.

And that's not so unusual. In the developing world, open dumps are the most common way to dispose of the rising tides of waste that accompany economic growth. Some are small. Some, like Reppi, are vast, rolling landscapes of trash, home to recycling businesses, ad-hoc housing, children, livestock and swirling clouds of dust, plastic bags and opportunistic birds.

Such dumps loom as one of this century's most pressing health and environmental challenges. According to the World Bank, cities will generate 2. 2 billion tons of solid waste by 2025, up from 1. 3 billion in 2010. Open dumps, typically located on the fringes of urban areas, are the primary destination for that trash. This leads to a variety of potential hazards, including water pollution, loss of biodiversity, increased greenhouse-gas emissions, and the spread of mosquito-borne illnesses such as malaria. Accidents like the one in Addis Ababa are only the most high-profile danger.