Satellite Beach, Florida, USA.
What appear to be rocks along Florida's east coast are actually living reefs built up over time by tiny marine bristle worms of the family Sabellariidae. The Sabellariid worms build protective tubes out of sand, ultimately forming large colonies that grow into mounding reefs. The reefs sometimes are exposed at low tide, creating tide pools and providing a habitat for a variety of other marine organisms. Please don't tread on the worms!