To make water drinkable, treatment plants weed out most harmful chemicals and bacteria. But the resulting drinking water is almost never pure H2O; even the most potable water contains a cocktail of dissolved chemicals that give it a unique flavor or cause slight variations in its pH. Researchers have found that the human nose is very sensitive to two molecules that often slip through the treatment process: geosmin (GSM) and 2-methylisoborneol (MIB). And while these molecules aren’t harmful in water, they smell earthy and musty, respectively, which can make water taste just gross enough to be off-putting. Water quality experts have to test water samples in a lab to determine whether these molecules are in drinking water, a process that is expensive and time-consuming.