UTH's and deep substrate are not a great combination in general. The deep aspen will basically insulate the UTH, meaning that heat doesn't make it to the top, and the surface of the glass underneath can get dangerously hot because all that trapped heat builds up. A thermostat will stop that from happening, but a UTH just doesn't make a whole lot of sense if you have deep substrate.
It's also more natural for heat to come from above, because burrows in the ground are generally cooler during the day (and more stable temperature overall) than the surface where it's warmed by the sun. So animals come out to the surface to get warm and dig to cool off (among other reasons).
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