Wachusett Dam is an engineering marvel in Massachusetts. You can only explore it twice a year.

The Wachusett Reservoir and Dam stand together in Clinton as one of the most ambitious and important infrastructure projects ever in Massachusetts. 

"It's 37 miles of shoreline, it's 65 billion gallons of water," Department of Conservation and Recreation education coordinator Katherine Parent said. "Water that's essential for about 40% of the people living in Massachusetts."

The crystalline waters of the reservoir provide drinking water for more than 3 million people in 53 communities in eastern Massachusetts, fed by a subterranean, gravitational aqueduct. 

Holding it all back, the reservoir dam - the world's largest hand-laid dam. 

"They started excavation for the dam in 1900, and they didn't finish it until 1906," Parent said. 

She knows the dam from side to side and top to bottom.

"The dam is 1,423 feet including the abutments," she said. "The interesting thing about the dam is when you see it, you're only seeing half of it."

The dam is just over 200 feet tall, extending 112 feet below ground to make it sturdy enough to hold back all that water. Standing on top of the central span, you can really appreciate its fortitude.

The only problem is, it's almost always closed off to visitors due to water supply security and public safety concerns. It opens to the public just twice a year - once on Mother's Day weekend in the spring, and again on Columbus/Indigenous People's Day weekend in the fall.

"Dam Day" is on Sunday, Oct. 11 from 10 a.m. to 1 p.m.

"We open the gate, and allow people to walk across and meet family and friends and neighbors and take pictures and enjoy the scenery," said Kelly Freda, DCR's regional director for Wachusett-Sudbury.

There are rules that need to be followed, like no pets and no bags allowed. Parking is available at Clinton High School, which is an easy mile-long walk away from the dam.

"It'll be a great dam day," Parent said.