We are looking down at multiple levels of walkways, balconies and handrails leading down to the bottom of power station. These are the areas that provide access to the wheelpit, penstocks and turbines below ground.

Down Towards The Wheelpit Nov 23, 2006

This is looking down at the multiple levels below the surface which lead to the turbines, wheelpit and tailrace of the station. Unfortunately I never had a chance to venture down and see any of it. However...

I highly recommend you visit Michael Cook's Rankine Generating Station page. He's photographed the underground levels substantially and provides detailed history and explanation. It also doesn't hurt that his photography is excellent.