Alien Invasion

Looking like something from a sci-fi film this is the seed head of Kingcup (a.k.a Marsh Marigold or Caltha palustris if you are posh). It's a member of the Buttercup family as is apparent when you see the bright yellow buttercup-shaped flowers.

It grows in wet boggy places and along stream edges, I use it as a marginal plant in my pond to try and keep the cats away from the fish.