Q:

Subsurface data in Google Earth

Are there any known hacks to get subsurface data into Google Earth? 
Sub-ocean-surface works with the release of Google Ocean, but you cannot go below the sea floor. 
The other major limitations of Google Earth (no vertical data, single placemark balloon) have hacks to get around them (thin buildings, custom icons).  I'm hoping someone has a subsurface hack. 
Edit: 
Possible approaches might be 

Find the default image and replace it with a transparent PNG
Capture the network requests to download tiles and respond with transparent PNGs
Convince GE to set the Level of Detail (LOD) for the primary layer to some negative or infinite value. 

On OS X, I think the following folder/code looks promising:
cd Google Earth.app/Contents/MacOS/shaders
$ ls atmosphere_ground_sun_off*
atmosphere_ground_sun_off.arbfp1
atmosphere_ground_sun_off.asd
atmosphere_ground_sun_off.ps_2_0
atmosphere_ground_sun_off.arbvp1
atmosphere_ground_sun_off.cfg
atmosphere_ground_sun_off.vs_2_0

Those files appear to contain assembly code instructions to the graphics card, and there is mention of "groundTexture", "groundRayleighMap", and other possible hack-able things. 

A:

I don't think this is possible right now.  Your data can be set to subsurface coordinates but you won't be able to view it through the current terrain surface (and AFAIK, that goes for GE Free, Pro and EC)
fwiw, this has been discussed on the GE forums before, maybe it will eventually make it into the product

