there are some spots in North Jersey that are large and explorable with varying degrees of "legality" most are public domain and i don't like to encourage anyone to offroad anywhere that isn't sanctioned . . . but there is no damaged to the environment in those areas.
“Stagg Hill” is located in Mahwah, NJ (GPS: 41.119887, -74.188701).
Not sure of the legality of the place, but I have seen a lot of 4x4’s, dirt bikes, and ATVs there on some occasions.
The area looks like a "right of way" for a natural gas company. There are very wide and long segments of land which have been cleared of trees, and consists of rocks/shale only. There are also wood trails. However, the area has some abandoned vehicles, a jet ski, and miscellaneous large and small trash.
Riding there definitely does not cause any environmental harm (other than chewing up your tires).