They are either 12, 14 or 16 inches wide, they live just outside our doors, and they come in a variety of striking designs. So why has no one heard of coal holes? Local historian Amir Dotan explores the streets of London to find the small metal discs you may not have noticed before, but may well have stepped over thousands of times.

James Ward introduces another curious talk about a subject that may seem boring, but is actually very interesting.... maybe.