The 23 year reminiscence of the SN1987A

Twenty three years ago on February 23, 1987, the explosion of the SN in the L.M.C. was observed both optically and by underground detectors. The optical observations were done in Chile and Australian observatories while the neutrino burst was detected by several underground experiments in the Northern Hemisphere, running at that time: Mt. Blanc in Italy, Kamioka in Japan, and Baksan in Russia and IMB in the USA. For the first time in the history of human existence, an astrophysical phenomenon has been observed in underground detectors. In this astrophysical event, the Mt. Blanc experiment detected five pulses on-line that were not at the same time, as detected by the other three detectors around five hours later. It is still not clear to astrophysicists why two bursts at two different times have been detected and how an SN can generate two neutrino bursts. After 23 years a model has proposed an explanation for a double stage collapse at two different times, as recently suggested by V.S. Imshennik and O. Ryazhskaya. In this paper, a detailed occurrence of something strange that happened on February 23rd is presented while most of the scientific information has been exhibited in other published papers.