Feast of St. Januarius
A sealed glass vial containing a dark unknown substance, allegedly the clotted blood of San Gennaro (St Januarius), is shown several times a year to a packed crowd in the Cathedral of Napoli (Naples). Whilst the container is being handled during a solemn ceremony, the solid mass suddenly liquefies before everybody's eyes. [1, 2]
This well-documented phenomenon is still regarded as unexplained  by believers and sceptics alike. Noted parapsychologist Hans Bender defined it the paranormal phenomenon with the best and historical documentation;  physicist Enrico Fermi seems to have expressed interest as well.
It is also one of the few recurrent non-medical, physical "miracles" that might be studied scientifically.
Above from CICAP, the Italian Committee for the Investigation of Claims on the Paranormal