Hydrogen can be stored underground just like natural gas. The cost is very low, particularly if there are salt domes available where large cavities can be solution mined. For example, the salt formation at Delta, Utah has room for 100 such cavities that could store enough hydrogen that (if burned in combined cycle power plants) could power the entire US grid for 30 hours. The estimated storage cost is as little as $1/kWh of storage capacity (power related costs are extra).