What is special about the underground structure known as eternity in the novel, The Third Policeman?

While you are in eternity, time does not pass and you do not change. The policemen's beards do not grow while they are in 'eternity,' and their cigarettes do not get shorter as they smoke them. Sergeant Pluck and Policeman MacCruiskeen take readings off of different meters while they are down in this structure, and when the readings look dangerous they must rush back to 'eternity' to make sure there is no explosion, which involves making some tricky calculations on the spur of the moment. Inside 'eternity,' anything can be manufactured out of nothing, but you cannot bring anything out of the structure.