Andy Clews writes:

Thus spake Andy Clews unto the assembled multitudes:
I wouldn't have thought a compost heap would get hot enough to smoke :-)

They do give off a lot of water vapour though (not quite steam).

...followed by various postings asserting that heaps CAN catch fire.

Blimey! I stand corrected. I've heard of Spontaneous Human Combustion
but I'd never heard of Spontaneous Compost Combustion before :-)

There's a sawmill on the Devon/Cornwall border that after tipping all
the sawdust in the same heap for a century or so has trouble with
digger drivers who don't pay attention when told not to try digging
through it. The core is so hot it ignites if exposed to the air, there
was talk of using it to heat the sheds but I don't know if that ever
happened.

In the surrounding woodlands the effect of the fallout plume from the
arsenic refining chimney is still visible from the air 150 years after
the mine closed down...

Anthony