One of the 50ft diameter diversion tunnels from the spillway on the Arizona side of the dam.
This tunnel drops about 600 ft to the river below and is capable of clearing water at
200,000 cubic feet per second. The water reaches a maximum speed of about 120
miles per hour as it exits into the river below. I would love to witness that some day.

This is a merge of three images for HDR.

More info can be found here: http://www.usbr.gov/lc/hooverdam/faqs/tunlfaqs.html