Here is a few interesting articles about this subject.

"We have tunnels for cars, for subways, electrical cables, and the internet. Could there be subterranean infrastructure for cows, too?"

Why was it covered up and still denied about? If there isn't some kind of conspiracy around these tunnels why would they be secret? There are plenty of other known to be dead or "Secret" tunnels in NY that are much larger and still accessible.

Is it because they are still in use in Manhattan? Check out the amount of huge cellars and sub-cellars being built in NY recently.

Type into google,

"cellar and sub-cellar" SQF manhattan

Even better the search

"below grade" "cellar and sub-cellar"

Is almost exclusive to NY.