That's right, wet hay bales can and will spontaneously combust. It sounds counter-intuitive, but wet hay bales are more dangerous than dry hay bales, because hay actually heats up as it dries. As the hay gets hotter, it starts giving off flammable gas. The heat it is generating combined with the gas it is giving off means they could light on fire at any moment if they've been stored before fully drying.
And where is hay commonly stored? In a barn, with an attached grain silo. And that grain silo is sometimes full of fine dust. Which, like flour, can easily explode at the smallest spark or flame.
This phenomenon is easy to exploit actually. Laying out "sheet mulch" with 9-12" of hay on top of an opaque layer (typically newspaper or cardboard) can greatly accelerate the production rate of soil and repair degraded soils. No tilling or digging required, the mulch retains water and cuts down evaporation, and weeds are smothered by the opaque layer. This yields high quality produce without fertilizer and with a minimum of labor. The late Bill Mollison of permaculture design describes it here (@21m): https://vimeo.com/142711756
In the interest of public wellbeing, from now on I shall restrict my vocabulary on HNews to Python keywords only.
Similar to explosives, the more thoroughly mixed it is the more energetic the reaction. Pouring LOX over charcoal and soaking charcoal in LOX are very different (the first ignites vigorously, the latter explodes like a stick of dynamite). http://www.popsci.com/diy/article/2009-09/most-dangerous-way...