Rumors of a Secret Government Base under Lake Arthur at Moraine State Park seem to be just that

Lake Arthur a man made lake which was developed in the early 60's is largest man made lake and 3rd largest state park in Pa.

https://en.wikipedia.org/wiki/Moraine_State_Park

It was built on what was scarred and damaged land from coal  and lime stone mining  and included some farm land .In fact there was a small pond thats part of the lake as well as an old section of US422  and a railroad under it. plus there are several old cemetary about the park  since it was aquired land.

https://clcing.blogspot.com/2011/03/lake-authur-and-old-422.html

While the lake has a rather bad ju- ju feeling for some people and claims of Big Foot  and UFO encounters and some drownings of individuals who went out on their own  to hunt and fish  .  One of the rumors all these years is that when the lake was being built and all the construction was going on that the government supposedly built a secret faculty under the lake.Using the construction as a ruse

The rumor was started supposedly by one of the construction workers who while drunk one night at a local bar spilled the beans on it and suposedly his work buddys shut him up about it. Which is one of several versions i have heard on the suposed base under the lake .

As you can see the lake is gigantic and goes all over the place  so if you where going to hide something you very well could do it especially with as thick as the woods are and all the swampy areas . But as far as I can tell its nothing more than a rumor like many urban legends you hear about mysterious places.  One of the amazing things is the lake filled at all with all the coal mines in the area so when they built the lakes foundation they did a good job sealing off the old mines which once again leeds credence to the so called underground base since the lake filled faster than expected. But thats what good construction work produces good results. As far as I can determine this is just another urban legend of a story being passed down from person to person. I know as much time as I have spent exploring the lake I never found anything indicating any kind of underground structure.
or even reports of mine subsidence which would indicate digging.