Transcript for People are searching for elusive treasure a multi-millionaire claims he hid
Reporter: Somewhere in the vast rocky mountain west, near a stream in a forest, or at the edge of a meadow, people are hunting. Wait. He could have stashed it under this end. Reporter: For a legendary supposedly hidden treasure worth millions to whoever finds it first. Let's go this way. Oh my god! I see bronze! Reporter: Dale nightel and Cynthia mechum among perhaps thousands looking for this bronze chest, said to be filled with a bounty of gold and rare artifacts. But only one man says he knows where it is. Here's a wonderful artifact. This is a lantern. See the wick? Reporter: Forrest fen is an 87-year-old former military pilot turned wealthy antiques dealer. He claims in 2010 he hid the treasure somewhere in the rocky mountains. Why would you do that? Well, in 1988 I was diagnosed with what everybody thought was terminal cancer. Decides that, we were going into a recession. And I just wanted to give some people hope. Reporter: Fen's health recovered. Going inside the vault here. Reporter: The idea of creating a modern-day treasure hunt stuck with him. There's 265 American gold eagles and double eagles. There's ancient Middle Eastern gold coins. There's hundreds and hundreds of gold nuggets. Two of them as big as a hen's egg. Reporter: He says even he doesn't know what it's all worth. Writers have appraised it between $1 million and $5 million. Reporter: He estimates around 350,000 people have taken up the search. He says, to find the treasure -- Begin it where warm waters halt and take it in the canyon down. Reporter: All you have to do is solve nine clues in his cryptic poem. Not far but too far to walk. Put in below the home of brown -- Reporter: The treasure could be hidden anywhere, in literally tens of thousands of square miles of territory. Anywhere from New Mexico to here in Montana. That, of course, assumes it exists at all. Some say it's a hoax. The story is real. The treasure is real. The treasure is hidden where I put it. Can you take me out there and show it? No. It might not even be here in New Mexico. Reporter: Today, thousands of online forums, videos, and documentaries like "The lure" fuel speculation as endless as the landscape. Obsessing over fen's every word for the tiniest clue. There will be no paddle up your creek -- I get between 100, 120 e-mails a day. Reporter: There's even the occasional death threat. There's one guy called me, he said, tell me where the treasure is right now or I'm going to kill you. Reporter: So far, nobody has found it, including Dale, who runs a website dedicated to finding the treasure. How many trips have both of you made trying to crack this one? It's somewhere in excess of 65 different trips. I've made over 100 trips. Reporter: But this time Dale and Cynthia are convinced the first clue -- Begin it where warm waters halt -- Reporter: Leads to the yellowstone national park area, known for steaming hot geysers and pools. We're going to start where warm waters halt. And for us that's Madison junction. Reporter: It's all a place Forrest says he spent many Summers as a kid. And I really believe it's at a place that has sentimental value to him. Reporter: In September, Dale and Cynthia each drove hundreds of miles to check out their latest hunch, inviting "Nightline" along. When you find the treasure, how are you going to divide it up? Are we talking thirds-ies here? How about your crew? Exclude them, never mind, sorry. Fifths, whatever. Where have you brought us this morning? This is greeling creek. Reporter: Into the wilderness we go, looking for something the poem calls a blaze. If you've been wise and found the blaze, look quickly down, your quest to cease. I don't know what a blaze is. Neither do we. Something in the terrain, a big white rock, a white stone, a cliff, maybe even a mound of rocks. Reporter: But just a few minutes later -- To me, that's the blaze. Reporter: They say they've found it. Look quickly down. Quest to cease, but tarry scant with marble gaze -- What does that mean? It means, get your butt over thee and start looking quickly down off that rock blaze. And find the Hidy spot. Reporter: We venture deeper into the wild, we're not alone. Oh my god, there's a dead animal. It's a dead deer. That tells you there's a bear in the neighborhood? Who brought the bear spray? I do have some bear spray. Reporter: The potential for danger, bears included, is real. And recently people have been literally dying to find fen's treasure. In June, Paris Wallace, a Colorado pastor, found dead in New Mexico. His wife says they like to search together, but the last time he went alone. It was a great way for us to get out in nature, to have some quality time together. Reporter: In 2016, Randy bellu's body was found along the Rio grande river. He'd vanished six months earlier, his family says while treasure hunting. 31-year-old Eric Ashby went missing in Colorado. His friends say he was searching for fen's treasure too. Fen was in his 80s when he hid the treasure somewhere out here in the wilderness. He says he didn't put it anywhere dangerous. Still, there may be those who are taking too many risks to try and find it. That's why New Mexico's top cop has asked for fen to call the treasure hunt off. You had talked about giving more clues. Again, I call for you to pull it. I'm not going to give a clue help people find the treasure, I'm going to give a clue to try to keep them out of trouble. I didn't anticipate people would die searching for my treasure. In the back of my mind, it had to be logical that that could happen. Grizzly bears alone are something to think about. It weighs on you? A little bit, you know. I don't feel responsible, I don't feel like I'm to blame for any of those things that happened. Reporter: Back with Dale and Cynthia. You see the big Boulders over there on the ground? Let's see if there's any kind of a place he could have shoved that chest in underneath that. Reporter: No rock is left unexplored. This is even better. But it's not there. Reporter: There is no gold. But just as fen says he intended, the quest comes with other rewards. It's an adventure to go looking. Even if I was searching the same area maybe a dozen times, every day was a new start. It's definitely going to be found. Somebody's going to figure it out. Reporter: Fen says some treasure hunters have gotten tantalizingly close. 200 feet. I know exactly where they were. Because they told me. That's close. Not close enough. Reporter: Fen says this may be his last TV interview. I just don't feel like I have anything to say anymore. Reporter: So we had to ask. Is there any tiny little hint, any tiny little clue you'd like to leave us with? He's thinking about it. No. Okay. Well, I will give you a clue. Try to simplify, if you can. That's good advice. Have you secretly given us clues in this interview that we should go back and parse your words? I've said more than I should have said. Reporter: For "Nightline," I'm Clayton Sandell in Albuquerque.
This transcript has been automatically generated and may not be 100% accurate.