I've never used an underground heater. Hatchlings in the wild stay buried under leaf litter almost 24/7. Heat comes from above, not below. To keep the temp. up I use a heat emitter on one side and a long tube 5.0 UVB on the other side. Low light,warm humid and moist substrate. Meal worms are also very hard to digest for a hatchling. I'm sorry about your loss. It's so sad when a baby dies.