By Kristen Johanson

PHILADELPHIA (CBS) — With Mother Nature releasing her frozen grip on the Delaware Valley for a few days, emergency officials are warning people about the dangers of playing or walking on ice. And it’s the perfect time for rescue crews to train.

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Dressed in insulated wetsuits, about a dozen members of the Philadelphia Police Department’s  Marine Unit spent hours at FDR Park in South Philadelphia Wednesday, pulling each other from a square hole chiseled out of the park’s frozen pond.

“In our area we don’t get ice too often,” said Lt. Andrew Napoli. “So when we get a winter like this, and we have ice that formed, we want to take advantage of training and being able to save somebody and getting somebody out of the icy waters.”

Each played the victim, with a teammate – or rescuer – rolling into the chilly water with a rope to grab hold, then the group on land, pulling the pair out.

(credit: Kristen Johanson)

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Napoli says as temperatures drop, frozen water can be unsteady.

“The public needs to be made aware that they can’t be walking around the ice, because they are unfamiliar with the ice,” he said. “They are unfamiliar with how ice forms, how ice can be weak at times and strong at other times. You can have an area 10 feet in diameter; it can be 1 inch or 12 inches thick.”

So, he says, it’s important to understand how ice forms and changes.

“If in a lake, someone goes under, there’s no current to take the person. Hopefully, the person pops right up from where they went in,” he said. “With river ice, with the under current, there is a possibility that they go under and then flow with the current and don’t come back up that same hole.”

But the hope is, they’ll never need to put their training into action.

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“If ice looks safe, consider it dangerous,” Napoli warned. “Do not walk on the ice, watch it from a distance, watch its beauty from a distance, please do not walk on the ice.”