I seem to recall having read somewhere how hive cities are constructed. Perhaps the Guns of Tanith, or something. Anyways, the description was of a hive city that stretched miles into the surface, not off the surface. Obviously, hive cities are described as stretching miles into the sky, but the description in said novel played it off like an iceberg. The glimmering spire was where all the wealth was, but beneath it, miles more deep into the planet's crust and even stretching into the mantle (some planet's cores are deader than others, and the AdMech does have shielding tech against the heat of magma so they can have generators near the heat source without them melting... I cite Dark Mechanicum (HH Novel) for that), a hive generally was primarily under the surface.
I also recall in science classes, we were told (both in high school and university) that the sun's radiation keeps the mantle and core of our planet hot. Without the sun's energies, the mantle would slowly cool, and the core too would chill. If all the fluff about Holy Terra being wrapped in a thick blanket of industrial smog and nuke mushroom debris (the latter may not be canon) is true, then Holy Terra would be in perpetual winter.
The way I'd heard about the oceans was that the majority of the oceans were boiled away during vast and horrendous wars, combined with the fires of industry. So, logically, if all the liquid evaporated, and the world is covered now in perpetual smog, would it not logically dictate that what water remains will forever be ice? (Southern polar ice cap)
Lastly, running back to the description of hive construction. Given how cold Earth would be, figure in that a majority of humans living on earth never see the light of day or even the gloom of smog, because they live miles beneath the surface. Factor in the possibility their hives reach very near to the core of Holy Terra. Measure the diameter of our planet, and now figure in the surface area you could provide for living space by inserting honeycombed catacombs beneath each hive about halfway to the core...
Not only is it plausible that Holy Terra contains 100,000,000,000,000+ humans, it's probably more than likely.
Also, aren't all the grand architectural sight seeing spots of Holy Terra clogged with pilgrims from across the galaxy? Despite being officially populated by only thousands, consider how many vagrants must be living there on an unofficial level, traveling from holy site to holy site within these sepulchral tombs.
If you need a visual example of a human civilization that habitates near the core of the Earth, watch the matrix, and then envision that their civilization is only the bottom mile of a hundred-mile deep hive, with a much denser population to cubic footage ratio (the humans in the Matrix are being hunted to extinction, the Humans on Holy Terra flood the city in cess and pool).
Hope that helped.
