The specific cause of action in your case, I believe, is known as “inverse condemnation”.
Not being a lawyer, I will leave it to you to look up that term and decide whether it applies. (IMO, it does) And hence, I believe your best shot will be to contact some of your neighbors and see if they are of similar mind. Or, you could search for “inverse condemnation attorney Pennsylvania” and see what you come up with, then take those results to your nabes.
As for the undisclosed “known defect” on your purchase, you may ask your eventual attorney but I suspect that would be fruitless to pursue. But again, IANAL.
If your lot is 150 feet deep and 75 feet wide (numbers chosen for example) and the state wants the rearmost 30 feet of it, one calculation is that you want compensation for 20% of your lot. But the reality is that this taking negatively impacts virtually *all* of your lot; and not only diminishes your enjoyment of same, but impacts the resalability and/or market value of same.
Thanks for the response. Inverse condemnation will apply if the offer they make sucks (which it will). And you’re 100% correct about the impact to the whole property not being proportional to only the amount taken. If they took 10% of my property for a toxic wast dump, it doesn’t mean I only lost 10% of the value of my home, LOL.