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
No word yet on whether the blood liquified today or not.