The fortress

[Michaelaw]

http://www.redzonefilms.com/pics2/pod.jpg

[Mad Aussie]

An apt name for whatever that thing is! :)

Nicely taken as well.

[AntZ]

Looks like a naval mine. Cool find.

[Iguanasan]

If I'm not mistaken, it's the casing that surrounds a chestnut and protects it as it falls from the tree. So, "the fortress" is an apt name for a this very nice photo.