This site is fairly remote, in a small patch of light woodland in Horsham district.
A rusty discoloured line runs up a nearby tree trunk, providing a hint as to this structure’s purpose.
The main visible feature is a large buried chamber, which has collapsed under the weight of the earth above.
An exit, connected to the main chamber by a section of sewer pipe, is readily visible from the nearest footpath.
Don’t know what it is? Don’t worry - the people who built it didn’t know either.