Audio from a cave?!

This was one of those jobs that we that we will look back on and think "did that really happen?!". We had a call from one of our suppliers asking if we could provide audio for a fireworks display in Loughton, North London. So far so normal? When we asked for more details, we were told, don't worry you can put all your kit in a cave! So we arrived at this incredible mansion, and sure enough we unloaded and set up the speakers near the audience, with the amplifiers and mixer placed in a cave a level below on the grounds of the house!