A piece of history was recently found under the streets of Portland. Pictured here is a section of a wooden water main, a.k.a. a hollowed out log. Firfighters would have to dig a hole to reach the pipe, then drill through the wooden pipe to intentionally create a leak. The hole would fill with water so the fire engines could pump it out. 

After the incident the firefighters would seal the hole with a wooden plug, hence the term "fire plug". 

Photo courtesy of Portland Water District.