Earlier this year, there was an art installation in the Turbine Hall called Shibboleth. It looked as though, somehow, the artist had managed to gouge open the floor to create a giant 548ft long fissure which ran throughout the hall and was up to 3ft deep in places.

I remember reading about it in the paper (it became infamous because some visitors actually fell down into the crack) and wondering how on earth it had been created. You can see pictures of the crack and the secret behind its creation here

When I visited recently, the installation was long gone but traces of it remain. They decided to fill in the crack with concrete, so you can still see where it used to snake across the floor. Even filled in, I thought that the scale of it was impressive and wished that I had gone to see it earlier.