I'd never even heard of this, but it's even got its own website: https://www.thebalticway.eu/en/history/

Or, here's an AI summary of the Wikipedia article:

The Baltic Way was a peaceful protest that took place on August 23, 1989 involving about 2 million people forming a human chain spanning 690 km across Estonia, Latvia and Lithuania. It marked the 50th anniversary of the secret Molotov-Ribbentrop Pact between the Soviet Union and Nazi Germany dividing Eastern Europe into spheres of influence. The protest highlighted growing independence movements in the Baltic states and solidarity against Soviet occupation. It represented one of the earliest and longest unbroken human chains in history. Notably, Lithuania became the first Soviet republic to declare independence just 7 months after this demonstration.