The largest pre-modern stone Buddha in the world was built as a piece of hydrological engineering wrapped in an act of faith.
The problem. At Leshan three rivers meet, and the confluence was violent, boats were wrecked and lives lost in the currents. Around 713 a monk named Haitong began carving a colossal Maitreya, the Buddha of the future, into the cliff above the water, on the belief that the Buddha's presence would calm it.
The engineering. There was also a physical mechanism. Cutting the seventy-one-metre figure sent an enormous volume of rock into the river below, altering the currents; and the statue itself was given a concealed drainage system, channels through the hair, the collar and the back, to carry off rainwater and slow the erosion of the soft red sandstone. That system is much of why it has lasted more than twelve centuries in the open, after the great wooden pavilion that once sheltered it decayed away.
The patience. Haitong is said to have blinded himself rather than surrender the project's funds to officials. Work stopped at his death and was finished by later patrons around 803, about ninety years from first cut to completion. The water below is calmer now; the tradition credits the Buddha, and engineers credit the spoil, and both are recorded here.