It's a layer of hydrogen sulfide, a toxic gas created by bacterial colonies and ... leaving high concentrations of iron in the stalagmites and red stripes visible in the sediment of cave walls.

Sulfur in your water supply is easily recognized by its offensive odor. Hydrogen sulfide gas causes the "rotten-egg" or sulfur water smell. Hydrogen sulfide in water causes no known health effects.

Chemical composition of ancient rocks Iron sulfide can only exist in rocks that were formed before there was oxygen in the atmosphere. Rocks containing iron oxide can only form if oxygen is present.