Apparently there are many entrances to underground tunnels here are some:
New York City
There is an entrance to the tunnels in New York City "in the vicinity of Midtown Manhattan that can be reached through an abandoned elevator shaft
that only very few know about
The Nahanni Valley (Canada) Entrance
This covers 250 square miles in the southern end of the Mackenzie Mountains of Canada. It lies almost 550 miles due west of Fort Simpson on the
Mackenzie River of northwest Canada. Hot springs and sulfur geysers keep the valley warmer than the surrounding areas by about 30 degrees year -round
(the valley is above 60 degrees latitude), making it perpetually mist-covered. This valley is inhabited only by animals as people entering the valley
are usually found headless and quite dead. The Indian tribes of the area avoid this valley. (These tribes include the Ojibways, the Slave, the
Dogribs, the Stoney, the Beavers and the Chipweyans.) This valley is often referred to as "the Valley of the Headless Men.
The Liyobaa Cave Entrance
This was sealed off by Catholic Priests who believed it to be an entrance to "Hell." "The village of 'Liyobaa' or to translate, 'The Cavern of
Death,' was located in the province of Zapoteca, somewhere near the ancient village of 'Mictlan' or the village of the 'Underworld.
"The Cavern of Death was actually located in the last chamber of an eight chamber building or temple. This temple had four rooms above the ground and
four more important chambers built below the surface of the Earth. This building was located in "Theozapotlan," and the tunnel entrance led one
beneath a mountain.
The Maltese Cave Entrance
This entrance is located on the island of Malta, near the small village of "Casal Paula." (This village is built on the "Corradino" plateau, and
overlooks the capitol town of Malta, "Valletta," as well as Grand Harbour.) In 1902, workmen digging a well in Casal Paula fell into a subterranean
cavern. The well was being dug for a house on "Hal Saflienti," the main street in Casal Paula. The cavern the workers had fallen into connected with
an entire complex of caves and tunnels. This entrance is known as the "Hypogeum of Hal Saflienti." (In Latin, "Hypogeum" is the name for an
underground structure.) "The tunnels under the Hypogeum have been sealed off even since a school took 30 students into the caves and disappeared,
guide and all. Search parties were never able to locate any trace of the people and children.
For more information visit this site
And here is some of the bases location: