This extraordinary time-lapse video filmed by the BBC for their "Frozen Planet" series shows how cold, sinking brine forms a deadly "brinicle" when it comes into contact with warmer waters below the surface.
Given the concept of "higher" or "purer" planes of existence mixing in strange ways, this is a nice example of how it works in real life. How an abstract plane of fire, or shadow, or evil can sink down to the material plane and wreck the life of the inhabitants... but not necessarily in nice spherical, circular areas. They can creep in strange ways across the 'floor' of our existence, leaving some areas mercifully unharmed.
God help those who accidentally cross its path to escape. I mean, think about what it's like for those starfish that manage to escape.. and those that didn't.