A Load Of Crap?
Certain types of manure used to be transported (as everything was back then) by ship.
In dry form it weighs a lot less. But once at sea - if water hit it, the manure not only became heavier, the process of fermentation began and released Methane Gas.
As the stuff was stored below decks in bundles, you can see what could (and sometimes did) happen. Methane began to build up below decks and the first time someone came below at night with a lantern: BOOOOM!
Several ships were destroyed in this manner before it was discovered what was happening.
After that, the bundles of manure were always stamped with the term: "S.H.I.T" on them
"Ship High In Transit." In other words, high enough off the lower decks so that any water that came into the hold would not touch this volatile cargo and start the production of methane.