Ever wonder where the word "shit" comes from? Well here it is:

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 water (at sea) hit it. It not only became heavier, but the process of fermentation began again, of which a by-product is methane gas.

As the stuff was stored below decks in bundles you can see what could (and 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 where always stamped with the term "S.H.I.T" on them which meant to the sailors to "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.