The Supreme Court will consider throwing out an $18 million penalty against Texas-based Southern Union Co. for illegally storing mercury at a rundown building in Rhode Island.

The justices said Monday they will hear the natural gas company's appeal of the criminal penalty that was imposed by a federal judge and upheld by an appeals court.

What makes the case unusual is that the company is challenging the size of the penalty under a line of Supreme Court cases concerning prison sentences.

Southern Union had used the building in Pawtucket to store outdated mercury-sealed gas regulators that it removed from customers' homes. The mercury was initially removed and shipped to a recycling center. But when that work stopped, the regulators and loose mercury were left to accumulate inside the building.