Turns out, those picturesque wooden water tanks that dot our city's skyline are holding more than just water and aggressively twee parties—there's also rodent carcasses, green goo, pathogen-breeding muck, insects, at least one mattress, a whole bunch of pigeons, unidentifiable biological growth, E.coli, and what one tank-cleaner poetically describes as "squirrel martini."

According to an extensive investigation published Monday by City & State, the ubiquitous rooftop water tanks, which supply drinking water to millions of city residents, are sometimes tainted with potentially hazardous material as a result of "widespread neglect."

While building owners are required by law to inspect and clean their water tanks, the report found that thousands of tanks go unexamined each year, and that the city is not even sure how many there are in total. Regulators, meanwhile, have reportedly issued "dramatically fewer" violations in recent years.

Those who have bothered to check in on the icky cauldrons have sometimes been horrified by what they've seen. Here's how City & State describes the tank providing water to downtown Manhattan officer workers with the New York City Department of Sanitation:

Shards of decayed wood are all that remain of the roof that once shielded the wooden water tank at 137 Centre St. On a recent morning, raindrops fell through the splintered support beams, rippling the surface of the dark water inside the open barrel. Something green grew on the sides of the rustic exterior. A shredded tarp hung off the top of the tank, like a tattered garment, flapping with each gust of wind. The holes in the tarp were so big that a bird could easily fly into the tank.

That tank and others tested positive for E. coli in 2015, but the city health officials who conducted that report later dismissed the findings as a "clerical error." A spokesperson for the NYC Department of Health and Mental Hygiene, which is responsible for overseeing the water tanks, told the outlet that they pose "very little risk" to New Yorkers, and noted that "there is no evidence that the water from water tanks raises any public health concern, and there has never been a sickness or outbreak traced back to a water tank."

Yet two senior scientists at the EPA told City & State that the tanks posed several clear health risks. "It sounds like a system that has got lots and lots of flaws to it and potential for public health impact," added a former EPA drinking water committee chairman.

Interestingly, shorter buildings aren't required to use their own tanks, as gravity accrued by the water as it travels from upstate reservoirs is enough to push it up to six stories. But buildings above six stories must provide their own tank—most of them are still built by two family-owned companies founded in the 19th century—and replace it every 35 years. While steel tanks are also an option, the wooden ones are widely preferred, because they're significantly cheaper and far easier to build and maintain.

A nifty map created by City & State using 13,000 water tank inspection reports shows the extent of the neglect. You can also search your own address, if you're interested in seeing what sort of horrors lie in wait within your water tank.

On the other hand: it's rooftop party season, and a home-brewed squirrel martini—best served with fish—might just be the summer's hottest cocktail.