Apparently the string of logging tools, each one measuring something different, needs some room to take their readings so the "rat hole" has to extend below the measured area.
Mr Berman told the Washington Post that although "billions have gone down a rathole in the past" in Pakistan, he did not want to "micromanage" the country's use of the new money.
Before leaving Adirondack, Spirit took images and collected miniature thermal emission spectrometer (Mini-Tes) data from the hole it ground with its rock abrasion tool (Rat).