Its surface of 194,000,000 km (75,000,000 sq mi, approx. 40% of Earth's surface area), implying a radius of around 4000 km (somewhere in between the Earth and Mars), was, with the exception of the Imperial Palace, entirely enclosed in artificial domes. It consisted of an enormous metropolis (an ecumenopolis) that stretched deep underground, and was home to a population of 45,000,000,000 (45 billion) human inhabitants at its height (although Second Foundation mentions a figure ten times that of administrators alone), a population density of 232 per square kilometre (600 per square mile, similar to the current population density of Germany or Connecticut). Its population was devoted almost entirely to either administration of the Empire or to maintenance of the planet itself, including energy provided by "heatsinks" (geothermal core taps) and production of food via underground farming and yeasts, as described in "Prelude to Foundation".