The Back River Wastewater Treatment Plant in Baltimore is home to more than 107 million spiders from over 31,000 species.

The Back River Wastewater Treatment Plant is operated and owned by the City of Baltimore. The facility is located on the west shore of Back River, which is a tributary of the Chesapeake Bay.

In 2009, managers of the treatment plants took assistance from entomologists, over an "extreme spider situation." The Baltimore facility spans about four acres and, in 2009, experts estimated that there were more than 35,000 spiders per cubic square meter. On an average a spider measured about 0.5 inch, which included its legs.

The experts found a spider species called the Long-jawed orb weaver, or Tetragnathidae, which is normally found in swamps. This spider species has shiny abdomens and long slim bodies. They typically use their webs for catching prey.

Experts who visited the treatment plant described the infestation as worse than normal and revealed that spiderwebs in some parts were so heavy that they damaged light fittings. In some places, workers cleared spiderwebs for accessing equipment. The spiderwebs were piled on the floor and appeared as thick as a fire hose.

The Baltimore facility was under operation despite the heavy spider infestation. However, the plant's operation and maintenance personnel raised concerns about potential spider bites.

The American Entomologist wrote in a paper that they were not at all prepared at the high spider population at the treatment plant. They found three dimensional as well as sheet-like webbing in the plant, which covered almost 95 percent of the facility's interior.

Albert Greene, one of the authors of the paper, revealed that this was the first time that the pest control team had witnessed a spider infestation of this immensity. The paper also described that seeing the heavy spiderweb was quite astonishing.

"On-site personnel should be reassured that the spiders are harmless and the facility's immense shroud of silk should be presented in a positive light as a record-breaking natural history wonder," read the paper.

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