Existence of Metastable states Do they (metastable states) exist naturally in atoms or they are artificially produced in them.
 A: A metastable state is an excited state which has such a long lifetime (time before it decays) that you can consider it "stable". That is you can assume the atom remains in that state for the whole duration of, say, your experiment.
Any excited state couples to the EM vacuum and hence wants to decay back to the ground state. The reason behind the long lifetime of metastable states is that the transition from them to the ground state is usually forbidden. That is, it is not allowed to happen via electric dipole transitions, which are the ones with the highest probability of occurrence. Therefore it has to happen via magnetic dipole, or electric/magnetic quadrupole or even magnetic/electric octupole transitions. These have gradually lower chance of occurring, which results in a long lifetime.
Of course, if atoms were left alone forever, they would decay back to the ground state. There hence has to be a process that puts them in a metastable state. This may happen naturally (as in, natural processes in stars or radiation from the Earth) or artificially, meaning we the humans need metastable Helium in an experiment so we create a plasma to send its electrons all over the place including the metastable state, which we then use for experiments.
