The Eucharistic miracle of Krakow relates to consecrated Hosts that emitted an unusual bright light when they were hidden by thieves in a muddy marsh. The thieves had stolen a monstrance containing consecrated Hosts from a church in the village of Wawel (outside of modern-day Krakow). They ultimately abandoned the monstrance and Hosts in a marsh outside of the village, where the miracle took place. The Church of Corpus Christi in Krakow, Poland contains paintings depicting the miracle as well as documents and depositions relating to the matter.
In the year 1345, King Casimar III the Great of Poland gave orders to build a church named Corpus Christi in honor of the Eucharistic miracle that was verified that same year. It had taken place in the village of Wawel, near Krakow. Some thieves broke into a little church (The Collegiate Church of All Saints), which was a short distance from Krakow. They forced their way into the tabernacle and stole the monstrance, which contained consecrated Hosts. They got away, but when they figured out that the monstrance was not made of real gold, they threw it (including the Hosts) into the muddy marshland that was prevalent in the area.
Priests from the church began a search, but they did not hold out much hope. When darkness fell, however, a mysterious light emanated from the spot where the treasure had been abandoned. Bright flashes of light were visible for several kilometers. Frightened villagers approached the area and reported back to the Bishop of Krakow. The bishop called for three days of fasting and prayer. On the third day, he led a procession out to the marsh. There, they found the monstrance, and within it they found the Hosts, which were unbroken and were the source of the unusual lights. The people began to pray and to celebrate the miracle. Annually on the occasion of the feast of Corpus Christi, the miracle is celebrated in the church of Corpus Christi in Krakow.