Question 1151 : No minors please

First of all, I'd like to thank Rohan for graciously accepting me back inspite of my continuing absence and use of profanities.
I was really touched, that is, until I read the "About Us" section. So guys, all my feelings for you are best described by the answer of the following question....

Earlier on, certain types of X used to be transported by ship.
When dry, it weighs a lot less, however, once exposed to water (in this case, sea water), it not only became heavier, but also began to ferment, thus releasing methane gas.
As X was stored in bundles below deck, methane began to collect under it.
Hence, the moment anyone lit a match, candle, lantern, etc. the whole vessel would blow up into smithereens.
Several ships were destroyed before the causative agent was discovered.
Thereafter, the bundles were marked X, meaning to sailors, "ships high in transit", the reason being that the volatile cargo be stored at a certain height in the ship such that no water came in contact with it.

Identify X. The EXACT word.

Answer: S.H.I.T