Newly-released underwater video shows an aging sewer line spewing treated sewage into the water less than a mile off the coasts of Virginia Key and Fisher Island near Miami.

A diver hired by the local non-profit Miami Waterkeeper captured the video. The group filed a notice of intent to sue Miami-Dade County Monday morning.

"My personal reaction is shock that this is still happening," said Kelly Cox, the program director for Miami Waterkeeper.

Cox said county officials have been aware of the problem since a fisherman spotted it last summer. She claims the county hasn't taken action to correct the problem.

A spokesperson for the county's water and sewer system told NBC 6 that the department assessed the area of the alleged leak in 2016 and didn't find any evidence of a leak at that time.

"The Department is committed to maintaining the County's water and wastewater system. We are working on making additional assessments on the outfall pipe. If in fact a leak were to be found, we would take the necessary steps to address it," spokesperson Jennifer L. Messemer-Skold said in a statement. "It is important to note that the fully treated water discharged from ocean outfalls meets all Environmental Protection Agency/Florida Department of Environmental Protection standards regarding the public's health and the environment."

The underground line is supposed to carry partially-treated sewage from the treatment center on Virginia Key and dump it about three and half miles out into the ocean. The apparent leak is less than a mile from shore.

Over time, salt water can corrode the aging pipes. The entire system is scheduled to be removed by 2025. Back in 2014, the county pledged to spend millions of dollars to improve how its system works.

"I hope they're encouraged to fix the problems that they find cause this is one issue that we're aware of, but who's to say how many other incidents are occurring, there might be other leaks," said Cox.