In January 2003, Tropical Cyclone Delfina dropped heavy rains over Malawi and Mozambique, causing devastating flooding. The floods lasted 47 days, killing 23 people and displacing some 400,000. More than a decade later, in January 2015, heavy rains swept through the two countries again, producing flooding in many of the same areas, killing nearly 300 people, and displacing just over 300,000.

These weren’t isolated events. The region experienced at least four other serious floods in the years between. But during that time, despite the relentless deluges, the total population living in flood zones grew. According to a new flood mapping tool called the Global Flood Database, between 2000 and 2015, the population exposed to flooding in Malawi and Mozambique increased by 150 to 250 percent.

It’s part of a concerning trend that’s occurring around the world. In a study published in the journal Nature on Wednesday, the architects behind the Global Flood Database found that during that same time period, the proportion of the global population exposed to flooding increased by 20 to 24 percent.

The study does not investigate the drivers of the trend, but there are a few possible explanations. Floodplains are expanding into new areas with dense populations due to sea-level rise and land-use change, but the researchers also think people are building at an accelerated pace in flood-prone areas. Beth Tellman, a geographer at the University of Arizona and the lead author of the paper, said that in many places, the cheapest land available to build a home is on the informal land market, in areas that are not zoned for formal development due to their high risk for environmental hazards. “That’s what we think is happening in some of the places that we’ve mapped,” she said, naming Dhaka, Bangladesh, and Guwahati, India as two examples.

Read the full article about flood-prone areas by Emily Pontecorvo at Grist.