There's an old powerhouse in my area that was converted into a restaurant. The website doesn't have many photos but it's interesting inside.

The plant was built in the early 20th Century by my former employer - Rust Engineering & Construction.

http://www.connorsgrillroom.com/powerhouse.html

http://www.seattle-tourist.com/images/gasworks-park-seattle.jpg

There were several abandoned mills around home that stayed as they were when shut down in the 70's. It was quite interesting to walk through places that were booming when I was school kid and got to tour some places. They suffered the same fate, being demolished for scrap when price got high a few years back. This was the same fate as lot abandoned mills prior to WWII when everything that wasn't red or nailed down went into these same mills.  
As far as tunnels, mines, until about 5 years ago one could enter the southern end of string of sloped iron ore mine shafts and walk approximately 17 miles north and come out looking down on the city of Birmingham, Alabama. Nearly all these mine quit working in 60's and it is strange how people and officials forget as everyone was dumbfounded when they were constructing as large sewer line through the mountain and hit the head shaft of one of the old mines. They killed the walking path as they didn't leave a path arounf the new tunnel and filled the area with raw sewage. The construction company had one hell of a time cutting the tunnel through the ore body. This particular mine was opened around 1870 and the old miners didn't seem to have any problem removing the ore.

I recently went by an old walking portal that very few know about and so it hadn't been  sealed off and as I approached I smelled sewer gas. This portal is kinda in the family as my father used it to carry water to the miners in 1900 at the age of 8.
 

mgtrp thank you for that interesting link. I am fan of old technology (posibly because started with electromechanical relay protections and now playing with numerical ones). I always enjoy to see old relays in some substation and to wonder what this should be intended for

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
 

There was an old oil fired powerhouse in downtown Seattle that was converted to Condos years back, Lake Union Steam Plant by name.  As an operator's helper in college, I used to love to go through the old plant while it was still in operation as a backup peak energy producing facility.  The old equipment was something else to see to say the least.

In another similar plant in south Seattle, Georgetown Steam Plant, some of the generator ID plates said "Not for use in aircraft", and were from the beginning of the 20th century.  Considering their sheer size and weight, that blew me away.

Mike McCann
MMC Engineering

I was part of a volumteer team that restored an old power house designed by Albert Kahn and built ny Henrey Ford in the 1930's. This building was scheduled for demo but our group saved it and restored it.

 http://www.milfordpowerhouse.com/1/165/index.asp

Be careful what you call abandoned.  Some of the pictures on that first link are of Con Edison's 59th Street Station.  It's still alive and well, providing steam to New York City.
John

The old downtown power station in Lansing, MI is in the process of converting to commercial space. There was a big to-do about evacuating the steam line because of loud noise. The building is prominent in the skyline; it is about 200' tall and 400' wide.