researchers used seismic readings to map what is beneath yellowstone .
partly-molten rock reservoir measures 11,035 cubic miles -lrb- 46,000 cubic km -rrb- .
it sits 12 to 28 miles -lrb- 19 to 45 km -rrb- beneath the national park 's supervolcano .
chamber is four times bigger than the magma chamber above it - but study said it is not posing any additional threat than it was before .