Earth as substrate heater?

snips of other messages...
>   I've been wondering about this.  Because the bottom of a lake/pond
>essentially has infinite insulation (depth to the center of the
>earth), doesn't this function like substrate heating?  On cold days,
>the ground is almost always warmer than the atmosphere (and
>eventually the water).  It seems that this could be very good 
>substrate heating given geologic time.
   Although in colder climates (like the US or Europe) the Earth is often 
warmer than the air/water above it during winter, in tropical climates, 
where our fish come from, in the absence of cold weather, the earth is 
cooler than the air/water.  Most tropical lakes (not rivers/streams 
however) are actually stratified thermally (meromictic) because the deep 
water, being much cooler, is more dense and remains on the bottom, not 
exchanging with the surface waters very much.  On average, the earth is 
about 56 deg.F (13 C) below the surface ( the temp of almost all caves 
around the world).  Groundwater and sediments in the tropics are 
predictably cooler than the overlying water/air.

steve petsch