Okay, imagine a really big, long metal tube that can go underwater like a fish. That's a submarine! Now, in 1966, the Soviet Union (which was a big country on the other side of the world) sent one of their submarines on a really big trip.
Think about how you might go on a road trip with your family, but instead of a car, it's a submarine going in the ocean. The Soviet submarine went all the way around the world, which means it traveled through lots of oceans and visited different countries.
But here's the tricky part. The Soviet Union didn't tell the rest of the world that their submarine was doing this. So, when the submarine popped up in different places, nobody knew it was coming!
Now that might sound like a fun, adventurous trip, right? But the reason it was a big deal was that the Soviet Union and the United States were in a cold war. That means they didn't like each other and they were trying to be the most powerful country in the world.
So, the fact that the Soviet submarine could travel all the way around the world without being seen by the United States Navy was a really big deal! It showed that the Soviet Union had a really good submarine program and that they could potentially sneak up on the United States.
Overall, it was a very important event in history, even though it was just a big trip that a submarine took.