On a day in January 1983 in Darwin, an idea was born.

It would take 70 driver-divers, a 60 metre snorkel and six months of planning.

"Like a lot of other projects in the Territory," Tim Proctor said, "it probably [came from] a few too many beers around the barbeque one afternoon".

"A few of us were discussing the possibility of driving a car across the harbour — not quite sure how it came up — but I guess that's where it started, too many beers."

Most hare-brained schemes cooked up around the proverbial barbeque, stay there. Not this one.

The vehicle gets some final modifications on Mandorah Beach. ( Supplied: Tim Proctor )

Mr Proctor and a team of others who were, like him, avid divers, keen four-wheel-drivers and a couple of mechanics, set out to build a car that could drive seven kilometres across the sea floor of Darwin Harbour, from Mandorah to Mindil.

And they did it — almost.

The first task was getting a vehicle.

"I guess we were testing the waters to see if everybody else thought we were crazy when we put it [a callout] around and asked if anyone would give us a vehicle," Mr Proctor said.

A local mechanic donated an old Landcruiser bull catcher, and the game was on.

"All of a sudden we had a vehicle and a plan," Mr Proctor said.

"We were contacted by media from all around the world."

Backyard workshop

The vehicle had a 60 metre exhaust pipe snorkel. ( Supplied: Tim Proctor )

The Proctors' backyard in the northern suburbs of Darwin was where the crackpot idea came to life.

"The backyard was our workshop then," Mr Proctor said.

"That's where we assembled this vehicle which started off a few hours on a Saturday and Sunday, which ended up being 24 hours around the clock, much to the disgust of my wife and our neighbours.

"There were quite a lot of modifications — probably the exhaust snorkel was the trickiest."

He explained that as the exhaust went up a 60 metre exhaust pipe that protruded above the surface of the water, condensation would form and run down to the engine.

The solution was to fit the starter motor with a complex seal system and pressure-compensating air system to keep it dry.

But, ultimately, the starter motor would be what eventually brought the underwater car to a stop.

Bar front mechanics

The planned route was about seven kilometres long, from Mandorah across the harbour to Mindil Beach, and 35 metres at the deepest point.

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"We knew where we going and we knew what was on the bottom," Mr Proctor said, 35 years since the big day.

"We needed to make sure we weren't going to drive off a cliff or into a solid wall somewhere, so we did spend a lot of time looking for a 'road', if you like."

The 60 metre snorkels and final modifications were installed on the beachfront at the Mandorah pub.

"In those days when the hotel was there, we actually had the welding machine in the front bar, the tubing was brought through the bar, welded together and passed down the beach and fitted to the vehicle before it departed," Mr Proctor said.

Deep sea driving

The vehicle was driven at depths between 20 and 35 metres, by 70 driver divers. ( Supplied: Tim Proctor )

More than 70 divers were involved in the actual driving, taking turns behind the wheel as the time a diver could spend at a certain depth expired.

"There were plenty of occasions the driver diver ran out of time and had to leave the vehicle before the new driver was there," Mr Proctor said.

"The vehicle ambled along the bottom of its own accord for minutes at a time, quite often. Fortunately its steering was fairly stable.

"[It's] pretty easy actually because you don't have to worry too much about getting bogged or getting in holes, the vehicle just sort of cruises along."

Between three and four kilometres into the trip, they ran out of air for the starter motor, cutting the mission short.

The vehicle then had to be floated to the finish line at Mindil Beach.

"We were expecting it when it happened," Mr Proctor said.

"I was very pleased we got as far as we did; we did better than three kilometres across the bottom.

"I was quite relieved. I thought 'that's done, time for another beer, I guess'."