An invisible part of the building, the crypts presents the history of the temple situated on one of the earliest inhabited sites of Vilnius, the development of the building, and the figures of outstanding merit to the state and Church of Lithuania who are buried there. The exhibition set up in the crypts and corridors introduces the funerary traditions and archaeological finds, and leads to the Royal Mausoleum.
The Royal Mausoleum beneath St. Casimir’s Chapel is the burial place of King of Poland and Grand Duke of Lithuania Alexander I Jagiellon (1460–1506), the wives of King of Poland and Grand Duke of Lithuania Sigismund Augustus Elisabeth of Austria (1526–1545) and Barbara Radvilaitė (Radziwiłł, circa 1522/23–1551), and the heart of King of Poland and Grand Duke of Lithuania Władysław IV Vasa (1595–1648). Their remains were discovered in 1931. The small crypt which served as a temporary burial site for the rulers’ remains for more than three centuries is also seen during the tour.
The crypts of the cathedral contain one of the oldest surviving frescos in Lithuania painted in the late 14th or early 15th century. It testifies about a clash between the traditions of Eastern and Western Christianity in the newly baptized country. In the center of the fresco there is the crucified Christ, while beneath the cross stand his mother the Blessed Virgin Mary and his beloved disciple St. John the Evangelist. The drawing itself and the Greek letters in Christ’s halo reflect the influence of Byzantine art, though Jesus is affixed to the cross with three nails, which is typical of Catholic tradition.
The Crypt of the Chapter, located beneath the presbytery of the church, is the largest and most picturesque space in the underground exhibition. This crypt is also the place where the sarcophagus meant for the remains of the most famous Lithuanian ruler, Vytautas the Great (circa 1350–1430), is preserved. Vytautas was the first ruler of Lithuania to be buried in Vilnius Cathedral. He contributed to the reconstruction and decoration of the church. In 1930, 500 years since Vytautas death was commemorated. It was hoped that by the time the remains of the grand duke would be recovered and transferred into the newly made sarcophagus. In fact, though, the search for Vytautas remains has not stopped yet.
The artefacts which shed light on the reconstructions of the cathedral and its decoration can be examined in the Crypt of the Chapter. One of the oldest items found so far is a small silver angel holding a shield with the coat of arms of the Vilnius Cathedral Chapter in its hands. The angel figurine, which was found in a burnt layer from the late 15th century, is thought to be a leg of an ornate reliquary that did not survive. A three-dimensional hologram makes it easier to imagine what the reliquary may have looked like.