I read, silage, the "Fodder harvested while green and kept succulent by
partial fermentation as in a silo" (WordWeb 3.02) was initially stored
underground. The gas released helps preserve the crop. It can also lead to a
low oxygen level in confined spaces and should be considered a hazard. A few
times, there have been reports a solo silo incident, where the farmer passed
out and even died. It is an interesting science, in agronomy, which I know
nothing else about. It was reported somewhere that archaeologists should be
aware of "underground silage",

George Myers