The place is north to Septimus(spelling?)post.It has a friendly frost atronach inside.I play that game one year now and its the first time i encounter that place.Which makes me wonder what other things i havent found....

And a question.Has anyone explored underwater Skyrim?I have found those ships in winterhold but i havent explored elsewhere.Considering how vast is the underwater section of Skyrim i am 100% convinced that Beth has some "hidden" secrets down there(like Fin glim on Oblivion or Easter eggs or even unmarked underwater caves).Still the place is so huge that i would spent hours to explore it....

If you want to find ALL unmarked places then buy the Official Skyrim Guide.

The cave you mean is meant to be as a refrigator i think.

OP I think the place you are talking about is place you go When you chose to go to jail in Winterhold.

Or check the http://www.uesp.net/wiki/Skyrim:Unmarked_Places or http://elderscrolls.wikia.com/wiki/Unmarked_Locations_%28Skyrim%29 pages.

Friendly Frost atronach! no way!

Yeah you are right.The Frost Atronach was friendly for about 30 seconds and then started attacking me.And when i came out of the cave 3 of his friends had spawned and were attacking me.I defeated them with the help of two horkers that happened to pass by... Anyway has anyone found any underwater secret?Like a sole skeleton corpse or an artifact?

Yep, but you'll want a waterbreathing enchantment. There's quite a bit of stuff lying at the bottom of Skyrim's lakes and rivers. There's even a sunken ship in the river up from Riverwood. What's in it? Dive in and find out.

It seems to be the one: http://www.uesp.net/wiki/Skyrim:Unmarked_Places#Jails