Story highlights CNN Hero Chad Pregracke has spent the past 15 years cleaning up U.S. waterways

Over that period, he has collected 64 messages in bottles

He's found lottery tickets, sheet music, people's ashes and even voodoo in the bottles

Do you know a hero? Nominations are open for 2013 CNN Heroes

They've been romanticized in movies and songs, but many people really do send messages in a bottle.

Chad Pregracke has found 64 of them, accumulating what he believes to be one of the world's largest collections.

The 2013 CNN Hero of the Year stumbled onto them while cleaning up the country's rivers . Since 1997, he and volunteers of his nonprofit, Living Lands & Waters , have removed more than 7 million pounds of debris from U.S. waterways.

CNN sat down with Pregracke to find out more about his unusual collection and what's inside each bottle.

CNN: How did you come to start this collection?

Chad Pregracke: Well, I found my first one in 1997, the year I started. There was a large flood on the Mississippi in 1993, and some guys put a message in a bottle and they threw it in the river and I found it. It had a flag on it. And I just thought it was kind of cool, so I kept it. ...

The second year, I found two or three more, and then I started to actually look for them. I try to keep as many as I can. ...It's pretty cool to find them because (for the most part) you never know how far they came, where they came from, who they came from.

CNN: How do you find them?

Pregracke: In my work, I see thousands of plastic bottles out there. But I'm always scanning, looking in every bottle for a piece of paper. You get psyched out a lot because people have stuffed napkins or wrappers in there. You get kind of excited and then, "Oh no."

Forget the treasure map fantasy. Chad Pregracke says many messages are just people's intimate thoughts.

It's the ultimate thing to find out there. When you're working, you don't use that time to open it right then. You put it in (the) boat, think about it and then open it up and read it. There's usually a lot of anticipation for it, especially at a big cleanup.

It's a romantic idea. And truthfully, most of them aren't like a treasure map. Usually they're more intimate. ... They're sort of writing to the universe or the one above, just putting their thoughts out there.

CNN: What types of messages have you found?

Pregracke: Oh, so many things. Some of them come hundreds of miles down the river or different rivers. ... Some of them have been found three or four times and passed on, and it'll keep going down the river and down the river. People will sign them and date them.

I've found lottery tickets, parking tickets, dollar bills. We did find a treasure map, but there was no treasure, I checked. I found one from some guys going to the World Series when it was in Kansas City; that was like 10 years old.

I found one from a girl who threw it in a stream in Lawrence, Kansas, and it made it to the Missouri River and then a couple hundred miles into the Mississippi River. That was cool. I found one for voodoo. It said, "You are bound now," and it had a string that pulled the paper tight. ...

JUST WATCHED CNN Hero: Chad Pregracke Replay More Videos ... MUST WATCH CNN Hero: Chad Pregracke 01:40

The newest guy on our board, he was at a cleanup for about 10 minutes and he finds this message in a bottle. It had wax at the top so it wouldn't leak and all kinds of sparkles and stuff inside. I thought, "This is going to be a good one." We couldn't see inside the bottle. I got a knife and opened it.

This stuff started blowing all over him. It turned out it was the ashes of somebody. It was like the movie "The Big Lebowski." But it was cool. There was a note about the guy who passed away, and it also had another note that said: "If anyone finds this, you are so blessed. Thank you so much." It was kind of neat that they decided to write a note to someone who might find it someday.

CNN: Have you ever gotten in touch with anyone who has sent a message?

Pregracke: Actually, there are fun ones who say, "If you find this, call me." I've called probably three to five of them over the years. Half of them forget they ever did it.

The one that I most remember was a truck driver. He was smart, because he put some rocks in the bottle, which weighs it down, which means the current will carry it versus the wind.

Anyway, his message gave his name and phone number, so I called him and said, "Hey, you threw a message in a bottle." And he said, "I don't know what you're talking about." And I said: "Dude, this is you. It's your name. You must've thrown it off -- my guess is somewhere near Burlington, Iowa -- and I found it about 90 miles downriver." And he said: "Oh my God, I'm pulling over. I did that five or six years ago!" He got superexcited. It made his week.

CNN: Do you think it's the world's largest collection of its kind?

Pregracke: It's the second-largest. I used to think it was the largest, but I just found out maybe a month ago that this dude down in Texas has 150 of them. He's a professor, and he has been documenting garbage on the ocean since 1978 -- not picking it up, just documenting it. So his collection is bigger. ...

But we have found way more than 64 actually. If volunteers find them and want to keep them, I let them, so we've probably found about 50 more. I just don't have them.

CNN: Do you have a favorite message?

Pregracke: I do. It's a song that was written for guitar called "Lavender Is You." It was thrown into the Wabash River and then went into the Ohio River. It's a love song. It's sheet music. ....

There's actually a lot of them that are pretty heavy, you know? They're not treasure maps, but they were written to a lost loved one. ... You don't want to keep it, because it was meant not to be kept. So you just kind of send those on, even though it feels weird letting a bottle go on.