Historical clue formulations

• One of two openings under a bridge — 2024-06-23
• Small breather? — 2018-04-15
• Blow hole? — 2016-02-26
• Hole in the head — 2011-11-20
• Frequently pierced place — 2011-01-23
• It may be flared — 2009-04-22
• Hole in the head — 2005-09-18
• Air intake site — 2001-03-02
• It may be flared — 2000-08-13
• Odor eater? — 1995-12-14