Here's why:

* Definition: Groundwater is water that exists underground in the spaces between soil and rock particles.

* State of Matter: Water in its natural state on Earth is a liquid at normal temperatures and pressures. Even when it's underground, it retains its liquid properties.

* Flow: Groundwater flows through these spaces, which is a characteristic of liquids.

While some groundwater may be frozen (solid) in very cold climates, or exist as water vapor (gas) in very small amounts, the vast majority of groundwater is in liquid form.