One of the apparent mysteries of physics is what the famous machinist Joseph Whitworth called the "adhesive force" by which two very flat objects will adhere to each other as long as the air clinging to them is squeezed out. This force can easily be over 100 PSI for steel blocks that have been well crafted.
According to the most recent sources known to me (eg, Krar, Steve F.; Gill, Arthur R.; Smid, Peter (2005), Technology of Machine Tools (6th ed.), McGraw-Hill Career Education, ISBN 978-0-07-723225-2, p. 98) the cause and exact nature of this force is unknown, other than it appears to be the result of molecules having some attraction to each other when brought in close proximity.
Has there been any progress on this problem in recent years?