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Calendenium was a naturally-occurring chemical substance. When combined with hydrogen it became highly volatile.

In 2367, an alien ship was trapped in a Tyken's Rift along with the USS Brattain and USS Enterprise-D. After the aliens telepathically communicated an urgent need for hydrogen, Data correctly deduced they intended to create an explosion strong enough to disrupt the rift by mixing hydrogen sent from the Enterprise with some other substance that could highly increase its volatility; calendenium was stated as one possibility. (TNG: "Night Terrors")