On a recent golden September Saturday, Lake Sakakawea gave up a mystery from its cold dark depths.

Divers and surveyors finally found the wreckage of a fighter jet that crashed through 3 feet of ice into 68 feet of water 35 years ago.

Members of the state's Professional Society of Land Surveyors have been looking for the F106 Delta Dart interceptor fighter on and off for 13 years.

Knowing they were zeroing in, a group went out Saturday in 75 degrees and light winds, an autumn day when boat traffic was virtually nonexistent.

They anchored a dive boat over a spot four miles northwest of the 4 Bears Bridge at New Town.

There was no red "x" like on a treasure map. There were only miles of green gray water in every direction. And there was a "hump" on the lake bottom they'd detected with a depth finder last summer.

A diver went down 35 feet. Nothing.

He went back down with a metal detector. It just about pinged his ears off.

The diver groped blindly in the silted muck, grabbed a solid object and surfaced.

From the boat, what he was holding up looked like a muddy stick. Once the silt and clay were rubbed off, the long object turned out to be a fuel line with an attached valve. It still smelled like jet fuel.

Bingo!

Divers went down again and again in zero visibility water, eventually bringing some 30 pieces of the plane to the surface.

At long last, they were holding proof positive. They'd found the plane that was last seen on March 10, 1969, in a colorful if ignoble episode in military history.

The pilot, on a training mission from Minot Air Force Base, rolled the plane, lost control and started heading down.

He ejected and parachuted safely through the late winter sky. The plane - $3.3 million worth of military hardware when it was built in 1959 - plunged through 31 inches of ice and sunk to the bottom. It's lain there ever since. The military said then the ice was too unstable to attempt recovery.

The pilot started walking and was eventually picked up by a rancher on a snowmobile.

The military investigated the crash scene, sent down divers to recover some equipment and took survey notes of the crash location.

The wreck was front-page news then, partly because it was the third crash of a Minot-based plane in five months. The other two crashes were fatal.

The story eventually faded, but not from everyone's memory.

Larry Smith, of Bismarck, is a former president of the land surveyors' society.

In 1991, he suggested that it would be an interesting project to try to find the plane based on the Air Force's original survey notes.

It was a fairly straightforward idea.

As it turned out, the surveyor's notes, which were supplied by the military, were not so straightforward.

"It was a mystery and a challenge," Smith said. "Putting a challenge in front of a bunch of surveyors is like waving a red flag in front of a bull."

Ken Link, of Hazen, also a surveyor, helped decipher the original notes.

"There were no electronics and no accurate distances," Link said. "There were fairly accurate angles, but not distances."

On several occasions, Smith, Greg Johnson, of Bismarck, and Link went to the area to try to retrace the original survey lines.

The original surveyor said he planted a wooden stake, or hub on a very sharp ridge and wrote, "I left a tomato juice can here also," Link said.

Another point of reference was a "white rock on a windy hill," Link said.

Some of the observation points had washed into the lake.

The most critical point of reference was "the west abutment on the 4 Bears Bridge."

Using the abutment still didn't get the surveyors over the plane wreck, though they got within about 1,000 feet in 1993. Divers sent down then came up empty handed.

Last year, Johnson reviewed the survey notes again and realized the surveyor was referring to a bridge pier out in the water, not the abutment, which is a bridge support structure on land.

It changed the whole picture.

With new calculations in hand, the three men went out last summer and set buoys. A depth finder revealed a sonar image of very likely "hump" on the lake bottom 80 feet from one of the buoys.

It looked good, but only by diving the location would they know for sure.

Smith also is a member of the Morton County Sheriff's Department's Dive and Rescue Team. He got permission to use the county's dive boat and equipment and solicited Morton County volunteers to make the dive.

The exercise would be good training for blind, cold water diving, if nothing else, Smith said.

Saturday's forecast was perfect. It also was possibly the last best chance for the season.

So they grabbed it.

Finding a point on the water based on last summer's survey calculations and depth soundings, it turned out "we were right over the debris point," Smith said.

When the first diver came up with a piece of fuel line, one of his diving buddies told him "I've never seen you beam like that," Smith said. "It was fantastic."

The debris is probably spread out in a 100-foot radius from the center, Smith said. The plane measured 70 feet long, with a 38-foot wingspan.

When it crashed in 1969, it was in 68 feet of water. The lake isn't as high now and having to dive 35 feet to the wreck made the task somewhat easier, Smith said.

Link said when the military turned over copies of the survey notes, they asked the survey society to supply them with a precise location, if they ever found it. Well, they did.

"We'll have to fulfill our commitment to the Air Force," Link said.

He said he met the military's original surveyor at a surveyors' meeting some years back.

He lives in Minot and when Link told him about the project, he pulled a little piece of metal out of his pocket.

"He carries a piece of that plane with him every day," Link said.

As it turns out, the pilot - Capt. Merlin Riley - is still living, too. He repairs vending machines in New Jersey. Smith said he didn't know if the former pilot would be interested in hearing the plane has been located. He said he didn't think he'd call him.

Smith said the mystery of the "missing" plane was solved through good research and good calculations.

"When I talk about this, I can say, 'Here's what math does. Geometry and trigonometry is how we solved this thing,' " Smith said.

For now, the surveyors won't reveal the precise location of the wrecked plane.

They plan to return next summer and create a precise map, which they'll make public.

"Then, anybody who wants to dive on it, can," Smith said.

(Reach reporter Lauren Donovan at 888-303-5511, or lauren@westriv.com.)