http://wikiless.i2p/wiki/Willis_Tower?lang=en
Contractors excavated the lot to a depth of 50 feet (15 m), and they removed 180,000 cubic feet (5,100 m 3 ) of dirt from the site. [ 25 ] By that November, Spencer, White & Prentis Inc. was excavating a trench around the site, measuring 60 feet (18 m) deep and 20 by 216 feet (6.1 by 65.8 m) across.