>I do wonder about the thermal wake of such submarines.
>But maybe the mass of the water is so large that it
>dissapears quickly.

Don't bet on it. A combination of the water bulge and temperature trail 
allows them to be tracked quite easily apparently. Satellites like the ATSR 
series can measure to very small temperature differentials (<0.1C easily), 
and there are some earth resources satellites that can measure sea height to 
very small amounts.

Now consider the water bulge you see in photos of modern nuclear submarines 
as they dive at low speeds, and then consider what is happening to the water 
around them as they travel at ~40 knots under water, and they can apparently 
be tracked with reasonable ease. 

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