From Halopedia, the Halo wiki
Tommy is prone to fantastic ideas, particularly "get rich quick schemes." He first tried to start an AI dog-walking business, then growing ginseng, and later moving on to sell duck meat from a pond in the backyard. At some period in time he also worked at an electronics store called McFarlane's. Anticipating the Covenant attacking and glassing Earth, he took his pension money and set out to build a bunker underneath the duck pond.
His plan was to use the water as a heat sink, as water can never become hotter than 100 degrees. Beneath the pond he installed a meter of ice, wrapped in piezo-foam and pelticours. Under that, he put a meter of wax, and finally a sealant of foamed glass. The living space was created from parts of the house itself, a fact that upset Rani when she came to visit from college. The shelter was outfitted with a year's supply of food, a composter to turn waste into soil, a starship-grade recycler, and a fusion reactor to power it.
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