The Leshan Buddha took roughly ninety years and three generations of work to complete, and it has survived over a thousand years partly because of a drainage system hidden inside the statue itself.
Ninety years, one project
Construction began in 713 CE under the monk Haitong and continued across the efforts of successive patrons and craftsmen after his death, not reaching completion until around 803 CE, an unusually long single construction span reflecting both the statue's scale and the intermittent funding and political support such a massive undertaking required across changing circumstances.
A drainage system nobody sees
Concealed within the statue's structure is a network of channels, gutters and drainage holes, cut into the hair, ears, clothing folds and other features, engineered specifically to carry rainwater away from the stone surface and prevent the kind of erosion that would otherwise be expected to badly damage a monument of this exposure and age over more than twelve centuries.
As this entry's companion article covers
The claim that carving the statue, and the rubble displaced during construction, altered the currents at the dangerous river confluence below it enough to make the waters safer for boats, the specific reason Haitong is said to have initiated the project, is addressed directly in this entry's companion article rather than repeated here.