The Maline Drop Shaft located near the Chain of Rocks Bridge is just like concrete sewer structures everywhere. They corrode when hydrogen sulfide gas is present. Moderately-high temperatures, long retention times, high biological oxygen demand (BOD) levels, and turbulence contribute to elevated levels of hydrogen sulfide gas which, in turn, provide the food for acid-producing bacteria.
The technical name for this process is Microbiologically Induced Corrosion (MIC). This 40-year old concrete structure was severely deteriorated from MIC. The upper portion of the walls were mushy with more than two inches of the original wall missing; in the lower half of this 50-feet-deep structure, more than five inches of the walls had corroded away