Actually, these secret tunnels were built in the 19th. century as part of the "underground railway" (you've all learned about that in US history class, right?). It stretched all the way from the deep South to Canada, and were built to help slaves escape. Of course, when the Civil War ended the slaves were freed, but due to the inclement climate in the North and in Canada, these tunnels were retained for general transportation and defense use.
Of course, today they are used for the purposes noted in the posts above.