1674 M. Lister Let. 7 Jan. in H. Oldenburg Corr. (1975) X. 425 When they strike into a Selfe Open, they most commonly meet wth cool air, wch alsoe cooles yt part of ye Groove immediately above it, after it hath gotten a passage into it, wch was warme before.

1684 M. Lister in Philos. Trans. (Royal Soc.) 14 512 That the Earth is more or less hollow is made probable, by what is found every where in the Mountains; viz. Natural Cavities or Chambers, which the Miners of the North call Self-opens.

1769 J. Wallis Nat. Hist. Northumberland II. 339 Towards the north end, is a cavern, very large, with an aperture at the top, usually called by the mine-men Self-Opens.

1824 J. Mander Derbyshire Miners' Gloss. Self-opens. Natural Fissures in the stone.

1961 L. D. Stamp Gloss. Geogr. Terms 346/1 Open, a large cavern (also self-open). [Also in later dictionaries.]

?1605 J. Davies Wittes Pilgrimage sig. X1 So He, (All being fulfild: the Sun obsurd [sic], the Earth, all, quaking, Graues self-opening, and, Natvres Frame dissoluing) Death, endurd.

1710 A. Evans Apparition 7 The Door, self-opening, took him thwart the Nose.

1855 P. J. Bailey Mystic 151 Towards the limits far of Faerie quick their anxious course they took, And the hill she entered first self-opened like a magic book.

1997 Marketing 24 Apr. 5/4 Blister packs. Why hasn't an entrepreneur devised one that self-opens on the input of a PIN?