In 1263 a German priest, Peter of Prague, stopped at Bolsena while on a pilgrimage to Rome. He is described as being a pious priest, but one who found it difficult to believe that Christ was actually present in the consecrated Host. While celebrating Holy Mass above the tomb of St. Christina (located in the church named for this martyr), he had barely spoken the words of Consecration when blood started to seep from the consecrated Host and trickle over his hands onto the altar and the corporal. The priest was immediately confused.
Initially, he attempted to hide the blood, but then he interrupted the Mass and asked to be taken to the neighboring city of Orvieto, the city where Pope Urban IV was then residing. The Pope listened to the priest's account and absolved him. He then sent emissaries for an immediate investigation. When all the facts were ascertained, he ordered the Bishop of the diocese to bring to Orvieto the Host and the linen cloth bearing the stains of blood. With archbishops, cardinals and other Church dignitaries in attendance, the Pope met the procession and, amid great pomp, had the relics placed in the cathedral. The linen corporal bearing the spots of blood is still reverently enshrined and exhibited in the Cathedral of Orvieto. It is said that Pope Urban IV was prompted by this miracle to commission St. Thomas Aquinas to compose the Proper for a Mass and an Office honoring the Holy Eucharist as the Body of Christ. One year after the miracle, in August of 1264, Pope Urban IV introduced the saint's composition, and by means of a papal bull instituted the feast of Corpus Christi.
“Eucharistic Miracle of Bolsena-Orvieto, Italy.” Real Presence Eucharistic Education and Adoration Association, 1987.
http://www.therealpresence.org/eucharst/mir/bolsena.html