A narrow and untrodden cavern at the bottom connects it with the outer sea; they could even then hear the mysterious thunder and gurgle of the surge in the subterranean adit, as it rolled huge boulders to and fro in darkness, and forced before it gusts of pent-up air.
Source: wiktionary
What all these passages are, of course I cannot say, but we thought that they must be the ancient workings of a mine, of which the various shafts and adits travelled hither and thither as the ore led them.
Source: wiktionary
It consisted of an opening in the forest which clothed the hillside with the black mouth of an adit in the midst of it, and a few big mounds of debris, beside which stood a rude log shanty.
Source: wiktionary
The Old Mine is currently entered through an upper adit as the main is no longer accessible.
Source: wiktionary
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