[Quix]

Should probably resist, but won't:

Catskill HILLS!

I live just south of Colorado. THOSE are mountains!

[Labyrinthos]

Although the Catskills are not the Rockies, they still provide a large area to hide things that go boom within close proximity to NYC. There are caves and caverns, old mills, underground storage facilities, abandoned camps and mines, etc. I have found roads and streams and even small villages (meaning a handful of buildings clustered together in the middle of nowhere) that don't even appear on local maps.

BTW, what makes the Catskill Mountains so impressive is that you can go from sea level to over 4000 ft. in a few minutes drive. In other words, when you look at the Catskills from sea level, they look almost as impressive as the Rockies viewed from Denver at 5200 ft.