How do underground water tanks work?
Underground poly tanks store rainwater in a buried tank fed by an inlet from your roof. Water is delivered to your taps or garden either by gravity or a submersible electric pump with optional mains changeover. The setup includes a distribution outlet, an overflow to stormwater to prevent flooding, and an access riser/drain for inspection and cleaning (often with a vent and level gauge).
Being underground keeps water cool and protected, reduces visual impact, and frees up usable yard or driveway space.