Puerto Rico is facing renewed danger in the wake of Hurricane Maria, as the Guajataca Dam fails under the strain of weather damage and heavy rainfall.
The government has called the situation "extremely dangerous", and the struggling dam is reportedly causing flash flooding in the island's northwest corner.
Two towns, home to some 70,000 people, are under threat and are being evacuated "as quickly as they can" because a crack has been found in the dam, the national weather service said.
Christina Villalba, who works for the island's emergency management agency, said officials believe the dam is certain to give way.
"It could be tonight, it could be tomorrow, it could be in the next few days... but its very likely it will be soon," she said.
Geologist Paul Nathanail told Sky News: "The water is swirling around quite powerfully and eroding the toe of the dam and undermining the entire structure.
"If the dam itself fails, then you'll see a catastrophic release of 10 or eleven billion gallons of water that will flow down the river, down the valley, leading to enormous devastation."
But as power is down, communications are proving difficult with people who live in the threatened areas.
And routes out of the towns are affected by flooding and debris after Hurricane Maria, which devastated much of the island. Petrol is also in short supply and there are reports of tension in queues for fuel.
Luis Gutierrez, a Democratic representative of Puerto Rican descent, said public health and infrastructure has been "literally blown away" by Hurricane Maria, which was a category four storm when it hit on Thursday.
He pointed out that existing problems in the US territory - including a recession, emigration and a lack of investment - has exaggerated the scope of the damage.
Felix Delgado, mayor of the northern coastal city of Catano, said: "Months and months and months and months are going to pass before we can recover from this."
Some residents said they were considering leaving Puerto Rico.
But Israel Molina, 68, said he would not leave his shop, which he has owned since buying and rebuilding it after Hurricane Hugo hit in 1989.
Across the Caribbean, 27 people have lost their lives because of Hurricane Maria, including at least six people in Puerto Rico.
Less than two weeks ago, Hurricane Irma brought extensive devastation, with winds of 185mph destroying buildings, flooring power lines and killing at least 84 people across the Caribbean and parts of the US.
Puerto Rico's 3.4 million residents were forced to traverse a maze of uprooted trees, destroyed scraps of buildings and blown-away cars on Friday.
With wreckage blocking streets and highways, police cars used loudspeakers to warn people to respect a 6pm to 6am curfew.
Assessing the gravity of the disaster from the shelter of a nearby family's house after her own roof collapsed, resident Alana Yendez said she had six barrels of water - enough to last her and 11 relatives for one month.
"This is an absolute crisis," she said.