Most water stored underground has been there for decades, and much of it has sat for hundreds, thousands or even millions of years. Older groundwater tends to reside deep underground, where it is less easily affected by surface conditions such as drought and pollution.

As shallower wells dry out under the pressure of urban development, population growth and climate change, old groundwater is becoming increasingly important.

If you bit into a piece of bread that was 1,000 years old, you’d probably notice.

Water that has been underground for a thousand years can taste different, too. It leaches natural chemicals from the surrounding rock, changing its mineral content. Some natural contaminants linked to groundwater age – like mood-boosting lithium – can have positive effects. Other contaminants, like iron and manganese, can be troublesome.