“Stay away or be swept away.” This grim advice was issued to the people of Puerto Rico by the US National Weather Service on Monday, after a dam overwhelmed by rainfall from Hurricane Maria developed a large fissure.

The Guajataca Dam is located on the Rio Guajataca in the north-west corner of Puerto Rico. The 90-year-old earthen dam houses a reservoir, 4 square kilometres in area and 4 kilometres long, which stores almost 40 billion litres of water. The reservoir is full to bursting, after Maria dumped 40 centimetres of rain on the surrounding mountains.

If the dam fails, it could cause life-threatening flash flooding downstream.

Since Friday, when the crack appeared, local officials have been warning of the danger and managing an evacuation. “We don’t know how long it’s going to hold,” Governor Ricardo Rosselló told the media on Sunday. “The integrity of the structure has been compromised in a significant way.”

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