Religious artifacts have been found buried under a primary school in the Lithuanian capital of Vilnius. These were found in the ruins of a 400-year synagogue. Archaeologists knew where the synagogue was located, but the site was not examined until 2015 when ground-penetrating radar was used to pinpoint the building's underground ruins.
Vilnius became a major Jewish city in the 14th century when the Lithuanian king gave Jews permission to settle there.The original synagogue was built of wood, but later the entire city was rebuilt in brick, including the famed Great Synagogue. The prestige of this synagogue was such that Vilnius was called the "Jerusalem of Lithuania."
After a 1748 fire, the synagogue was rebuilt, but city authorities did not want the synagogue to tower over the churches, so parts of the synagogue were built below street level. This is why it is preserved today. Read more here and here.
Hundreds of mummified human remains have been found buried in a crypt of the Dominican Church of the Holy Spirit in Vilnius. For most of their history, the corpses were preserved intact by the cool temperatures and ventilation in the underground chamber. These mummies have been studied by medical researchers to gain a better understanding of the health of the residents during the Middle Ages. Read more here.
Related reading: 17th Century Smallpox Retrieved From Mummy in Vilnus: In Search of the Jerusalem of Lithuania