from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A sound like that of distant thunder; may have seismic origins.
A brontide is a subterranean noise, resembling distant thunder and caused by a minor quake.
Often the gas bursts through the surface to give a natural atmospheric explosion called a brontide.
Apparently, then, the true brontides (many other have often been mistakenly reported as brontides) are only rumblings of earthquakes too feeble for registration or other than aural detection.
This word comes from the Greek 'bronte,' thunder, which also gives us 'brontosaurus.'