Historical clue formulations

• Pore, e.g. — 2013-02-02
• Opening — 2009-08-13
• Vent, e.g. — 2006-10-13
• Opening — 2006-04-02
• Hole in the head — 2000-02-18
• Opening — 1995-05-19
• Aperture — 1994-11-07