I had no idea what this structure was when I took the photograph recently. Mark and I had met up in Manchester and I had the intention of taking some photographs around the city centre. The light wasn't particularly interesting, so after a nice meal I drove home. On my way I knew I would be driving past this structure. As I drove the sky cleared and the evening sun shone down (typical as I was heading away from the action). I decided to pull up and walk around for ten minutes to try to capture an image of what I have recently learnt is an empty gas holder. When they fill it with gas the sides come up - clever stuff!