Bigger than we think? Underground Noises

I know about all the well-known supposed underground bases and secret lab locations (Area 51, Dulce Base, etc.), but is the government digging up a lot more than we think?

People across the globe have been claiming to hear strange noises from underground, including drilling, engines and humming noises.

Here's some articles for you:

www.science-frontiers.com...
paranormal.about.com... (half way down)
www.abduct.com...

What do you think?

we used to get humming coming from the ground when the channel tunnel was built, it runs almost directly under our house, at night time there would be a very quiet but constant low pitched hum, during the day we couldnt hear anything because of background noise from the main road

Possible solution:
Some people say there are alien tunnels built all across the world underground.
Theres lots on google.

EDIT: Or even government tunnels ^_^

[edit on 15-7-2006 by Majestic 12]