An impressive steel arch bridge spans the gorge on a massive underground foundation shaped by unusually difficult geology and rugged terrain. Before construction began, geologists and engineers studied the site for many years to find subsurface material capable of supporting the enormous forces of the bridge and its traffic. Although the basalt exposed in the gorge at first seemed strong enough, weaker layers lay beneath the hard surface. Each lava flow contained a solid middle section but much weaker, porous zones above and below, formed by gases trapped in the rapidly cooling outer portions of the flow. Over a long time, groundwater percolated through these porous zones and weathered them to a near-soil condition. The problem was further complicated because sagging of the Lake Superior basin had caused the rock layers to dip about 10 degrees to the east. Together, these conditions made the design of the 16 different footings that support the bridge an intricate problem. Subsurface core drilling and detailed mapping of exposed rock helped geologists predict the three-dimensional extent of the solid portions of the lava flows, although the work was made harder because adverse terrain prevented drilling at some foundation locations and the team also sought to preserve the area's natural beauty. After the solid and weak layers were carefully mapped, construction plans were completed and the bridge was built, with the subsurface predictions proving reliable and only minor adjustments needed during construction.