A metastable state has a higher energy than the ground state that a physical system can become trapped in for some length of time. A simple example is a ball sitting on a hilltop. The ball's energy would be lower if it rolled down the hill; but unless something disturbs it, it will remain where it is. Metastable states of atoms are put to use in atomic clocks because they are long lived, and therefore correspond to a clock frequency that can be known very precisely. In biological physics, valleys in the energy landscape correspond to metastable states, as do low-lying peaks in the fitness landscape.