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]