Hidden underneath Boston is a web of 800 wells.
"This well network is really the canary in the coal mine. This is the warning sign," said Christian Simonelli, executive director of Boston Groundwater Trust.
The well levels are so key because so much of Boston is filled in.
"All this Back Bay area, South End, has been filled in," he said while showing maps outlining the city's expansion hundreds of years ago.
Many buildings in those areas, constructed on the weak fill, are supported by wood pilings underground.
If that wood suddenly sits outside of groundwater, it can rot.
"We have to make sure the groundwater level is always above the tops of those pilings," notes Simonelli.
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Right now, the levels are dropping due to drought.
"It's really been a slow steady decline in groundwater levels since May," he said.
A property in the South End provides a classic example of what happens after prolonged low water levels.
"In some areas they had rotted away or were not exist any at all," said John Holland of the pilings under the building he is currently renovating.
That rot left the building's facade, which Holland hoped to preserve in a rehab project, unsteady.
"It drove out a change in the project, where we had to go back to the historical commission and get permission to remove the entire structure," he said of the challenges he faced. It's something he's faced in other projects in Boston as well.
The extra steps delayed the South End project by about a year, and cost money.
"The older fabric of these buildings in these neighborhoods is why people want to relocate down here so building a brand new building is, I believe, less valuable than a persevered building with new behind it," he added.
The rot in his South End building happened over a long period of time. And officials in Boston say it does in fact take years for damage to occur because of rot. So, while they continue to monitor levels during this drought, they're not worried about widespread damage in the short term.