We are blessed with a thalassogen on this good Earth
that knows not our rules, or has forgot ‘em
when most liquids get hot they rise to the top
when they cool they sink fast to the bottom
Our friendly Earth liquid tries hard to comply
As it cools it adds ever more weight
But just when it seems it’s about to freeze
It exhibits an irrational trait
It gets a bit lighter then freezes to ice
Which is wonderful news for us sheep
Imagine it did not and what’s the result
Well, ice would all sink to the deep
So what! you may say and little would care
But more ice would soon be forming
and sinking right down to lay on the rest
resisting all attempts at rewarming
It would not take long for the oceans to freeze
from bottom to top, not so nice!
Come Summer and maybe the top bit would thaw
and float on the heavier ice
No plankton, no fish so none of us here
could ever have come up from the sea.
So give thanks to the lightening that always occours
beginning below four degrees C.
Boadicea's Chariot 
I believe you’ll find that it’s one L and two Ss – thalassogen 😀
Ta. Bearsy, corrected, I knew the word, just didn’t know how to spell it! 😦
Conceptually interesting and a bloody good thing too!
Tundra/taiga always interested me too. Melting on the top, underlaid by frozen soil/rock and then presumably melted below that from the earth’s heat. One never hears about how deep the frozen layer is though. Presumably thinner where near volcanic activity etc etc.
You really are a pest, now I shall think about that for hours and end up seeing if there is any current research! Bugger off and go fishing!