The exact origin of this tradition is not unknown, but it is considered that the first crosses were placed after the 1831 uprising against the rule of the Russian Empire in Poland and Lithuania. Over the centuries, the place has come to signify the peaceful endurance of Lithuanians. When the old political structure of Eastern Europe fell apart in 1918, Lithuania once again declared its independence. Throughout this time, the Hill of Crosses was used as a place for Lithuanians to pray for peace, for their country, and for the loved ones they had lost during
Most recently, the site took on a special significance during the years 1944-1990, when Lithuania was occupied by the Soviet Union. Continuing to travel to the Hill and leave their tributes, Lithuanians used it to demonstrate their allegiance to their original identity, religion and heritage. It was a venue of peaceful resistance, although the Soviets worked hard to remove new crosses, and bulldozed the site at least three times.
On September 7, 1993, Pope John Paul II visited the Hill of Crosses, declaring it a place for hope, peace, love and sacrifice.
Reportedly there are more than 50,000 crosses of varied size - from three meters tall to the countless tiny ones hanging profusely upon the larger crosses, and variety - ranging from beautifully wood carved to metallic.
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Source of info: Wikipedia
Credits: I am thankful to Umme-Ayaan for suggesting this site.