[LukeL]

Seneca Guns, Mistpoufer. They have been heard for centuries and no one knows what they are.

[ifinnegan]

1850 short story by James Fenimore Cooper talked about this.

The Lake Gun

by

James Fenimore Cooper

“The “Lake Gun” is a mystery. It is a sound resembling the explosion of a heavy piece of artillery, that can be accounted for by none of the known laws of nature. The report is deep, hollow, distant, and imposing. The lake seems to be speaking to the surrounding hills, which send back the echoes of its voice in accurate reply. No satisfactory theory has ever been broached to explain these noises. Conjectures have been hazarded about chasms, and the escape of compressed air by the sudden admission of water; but all this is talking at random, and has probably no foundation in truth. The most that can be said is, that such sounds are heard, though at long intervals, and that no one as yet has succeeded in ascertaining their cause.”

http://www.gutenberg.org/files/2328/2328-h/2328-h.htm

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I assume these booms in Philly are more ground-based and localized.