PHILADELPHIA (AP) — A man checking out a sinkhole in a street saw something unexpected inside — a pool of what looked like green ooze.

Steven Reitz said Tuesday that the sinkhole opened in the Northern Liberties section of the city last week and the ooze appeared several days later. He said when he and some friends spotted the green pool, they didn’t know what to make of it.

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“We kind of walked over and looked and saw the neon green and said, ‘Whoa, look at that,'” Reitz said.

The hole had been boarded up by the time Reitz checked it again later that day, he said, adding that similar sinkholes appear every year and appear to be moving down the road.

Gary Burlingame, director of labs for the Philadelphia Water Department, said the coloring is simply dye, which helps workers determine the origins of sinkholes.

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Workers pour the dye into flowing water underground, Burlingame said, then go to a manhole down the street and look into it. If they see the color they poured into the system, they know the water is flowing in that direction.

Burlingame stressed the dye is safe, non-toxic and biodegradable.

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