In March 2005, the construction process began with the sinking of the first caisson for the tower into the bedrock. In April, construction began on the foundation below the Chicago River. In July 2005, water from the river began seeping into the building site, through crevices in a corner where the foundation wall meets the Wabash Avenue Bridge. Divers discovered that the leak could not be sealed from the water side. After several other failed attempts to correct the problem, they drove a steel plate next to the gap and filled the space between with concrete after digging it out.