Underglow has to be my favourite piece from Light Up Queen Street as it struck me as the most innovative and atmospheric. It's a simple idea, just put some lighting in the odd drain here and there.
There are several lit drains before Guildhall, but there are a number of them located along the full length of Queen Street. Once the night falls, these eerie lights give the impression of something going on beneath one's feet. It's surprising how many people don't notice them at all though. I've watched; most people seem to pass them by without even hesitating. How sad.