Diver Alex Lundberg spots something on the sea floor. What is it? A piece of wood?

Nope. It’s something far rarer—a mastodon tusk!

Mr. Lundberg and a friend had found fossils in the same place before. Mammoth teeth. Bones of an ancient jaguar. Parts of a dire wolf (an extinct canine). They also discovered small pieces of mastodon tusk, but nothing this big and complete.

“We kind of knew there could be one in the area,” Mr. Lundberg says. He brushed away sand covering the fossil. That revealed a large tusk.

The tusk might be as big as you. It measures about four feet and weighs 70 pounds. Mr. Lundberg found it at a depth of about 25 feet near Venice, Florida.

Mastodons are related to mammoths and current-day elephants. Scientists say the extinct creatures lived mainly in what is now North America. People across the continent find mastodon remains often.

Right now Mr. Lundberg is storing the tusk in a glass case in his living room. But he might not get to keep it. Florida laws say fossils of vertebrates found on state lands—or near shore—belong to the state. Mr. Lundberg has a permit to collect such fossils. But he must report the find to the Florida Museum of Natural History.

Mr. Lundberg thinks the museum will likely let him keep his find. No one knows how long it’s been rolling around the seafloor or where it came from. It might not be very useful to scientists. “It’s more of a cool piece,” he says.

The eyes of the Lord are in every place. — Proverbs 15:3