… because cars don’t float.
Some used to, at least for a little while. 
There were two famous instances of cars which plunged off of the Mackinac Bridge. See "Work and major accident fatalities in this article: (Mackinac Bridge - Wikipedia)
I remember that after the second incident there were questions about whether there might have been more fatalities - people who were known to be on a trip where they would be crossing the bridge at night and who never showed up at their destination. (Apparently the bridge authority was not keeping track of “vehicles on / vehicles off”.) The newspaper reports initially discussed some sort of magnetic method of detection. However the follow-up articles stated that this would not be feasible - metal objects were lost off or thrown off of the bridge during construction and they would give too many false positives.
You want to have some fun on YouTube, check out Shed-Happens. A repo-man repossess sheds. Everything from tiny sheds in trailer courts to barns, garages, and small homes. He has amazing tools and skills.
This was a major plot point in one of Sue Grafton’s Kinsey Milhone books. A missing person is found months after disappearing, and all that time the car has been undiscovered in a lake, just yards from the shore and from houses along the road.
Since there are people on the board still reading her books, it’s P is for Peril and the lake is mentioned on the first page. Kinsey just happens to be in a house above the lake where there’s a telescope and sees broken branches that show the car’s path into the lake
The guardrail doesn’t look 45 years old, either. Surely it’s been replaced a couple times since then. You would think the installation crew would have noticed the car.
Was this explanation already covered? There are many instances of it already visible in this thread:
Very extensive aerial photographs of populated land areas, including lakes, visible to large numbers of people browsing online. They can pan and zoom to their hearts content. And the images are occasionally updated with different sun angles. Also, “street view” imagery. These tools have only been available for 10-15 years, and get better all the time.
I have heard a few times recently that someone browsing online spots a dim outline under a lake in such a photo, leading to solving a long cold missing persons case. ISTR this happening at least a couple times recently. I even believe I’ve read such accounts online, perhaps even in this forum. But not yet in this thread.
Sometimes it’s in a river: a couple of teenagers who disappeared 21 years ago in Tennessee and found a few days ago:
Depends upon how the car enters the water. Vertically, water is harder to displace & is a harder hit; ever belly flop off a diving board? Though it doesn’t hurt when you walk/run in from a lake or ocean beach. Depending upon a number of factors, including speed, shorline obstructions (trees, rocks), terrain, angle, a car could make it past the edge of the water. even partially floating a little bit, possibly further out towards the middle before settling on the bottom, more like a rock that was skimming the water.
True. I’m part of a water/dive rescue team. A diver & tender pair do a grid pattern search where each pass is slightly less than armspan; depending upon water conditions there can be zero visibility under there, even at relatively shallow depths & the divers are basically swinging their arms back & forth over the bottom in an attempt to feel for something that shouldn’t be there (rocks, old tires, etc.). Think how long it would take you to search a small parking lot of you were lying down on a skateboard blindfolded, only able to swing your arms to find whatever you’re looking for & all directional changes are communicated to you by pulls (& confirmations) on a rope tied to you
In 2015 a funeral home in Michigan hired someone to install Christmas lights on a tall pine tree located on the business’ property. The tree is next to a pond. While installing the lights using a bucket truck, the contractor looked down and noticed a car was submerged in the pond. The car was pulled out and the remains of a body were found in the driver’s seat. It was determined the car was driven into the pond in 2006.
At the time, the submersed car was visible in Google Maps.
A Small town disappearance on April 3, 2000. The article says it’s emotional for police because they had known this couple (Erin Foster, 18, and Jeremy Bechtel 17) in high school.
I’m not familiar with this channel but Sides is doing worthwhile work. Bringing closure to a lot of people. https://www.newschannel5.com/news/white-county-cold-case-brings-new-answers-with-found-car
This was just reported a few days ago.
I lean toward the alien abduction theory myself.
This happens so often in northern states, and I’m sure Canada, it’s just a blip in the news.
It seems unthinkable now, doesn’t it? Now, people willingly put apps on their phones so they can be tracked by friends or websites all day long.
These turn up a lot on a reddit I follow and you are right about the explosives. It happens so often there is an automatic tagging to not handle the item and call police.
As an aside, I think I would actually like to give magnet fishing a try, but I’m not sure how long I would stick with it before boredom set in. It actually seems a lot like regular fishing so I assume you load up the boat/car with beer and friends.
Don’t leave us hanging, who was it? 
Now everyone knows what a white Pinto looks like after 45 years underwater. I wonder why the flat top parts rusted away but everything else was still more or less attached? Was it because it was upside down?
Good one! Point for you.
[quote=“mordecaiB, post:52, topic:955749”]
You really don’t know?
Sorry, I thought the smiley would be enough to convey intent. Yes, I knew.
Heh, I immediately thought of Wilbur Mills and Fanny Foxe.
Though, in that case, Foxe (an exotic dancer) jumped out of the car of Mills (a Congressman) while the car, which had been pulled over by the Capitol Police, was still on solid ground, and she then jumped into the Tidal Basin in an attempt to flee the scene.
Still a great story. 
I’d forgotten the details. Thanks for the refresh!
I had looked up the Mills/Foxe case a few weeks ago, for another thread here on the SDMB about political scandals of eras past; thus, the details were still fresh in my mind. 
Looking at the Google Maps street view, the creek looks smaller on one side of the bridge and bigger on the other. I am not a civil engineer, but I think it is possible that turbulence on the downstream side of the bridge excavated a big hole, which might even be seasonal depending on whether there has been much rainfall recently. The design of that bridge in particular, with the concrete-floored spillways, might even make it more likely to have a deep hole on the downstream side. Redneck wisdom is that swimming holes come and go in creeks like that, so you should always check the depth before you take a dive off a bridge.
Nitpick, Fanne and not Fanny Foxe Fanne Foxe - Wikipedia
She died early this year. The NY Times title for her obit:
Fanne Foxe, Who Plunged Into the Tidal Basin and Emerged Famous, Dies at 84
A stripper known as “the Argentine Firecracker,” she was at the center of a political sex scandal that rocked Washington in the 1970s. https://www.nytimes.com/2021/02/24/us/politics/fanne-foxe-dead.html
It’s paywalled; here’s some quotes:
Fanne Foxe, the stripper known as “the Argentine Firecracker,” who leapt from the limousine of Representative Wilbur D. Mills and plunged into Washington’s Tidal Basin after a night of drinking, exposing one of the biggest political sex scandals of the 1970s, died on Feb. 10. She was 84.
Her death was announced in a paid notice in The Tampa Bay Times. It did not say where she died or give a cause.
Until the Tidal Basin episode, Mr. Mills had been one of the most powerful members of Congress, an 18-term Arkansas Democrat who chaired the Ways and Means Committee and wrote major tax legislation. He had flirted with a bid for the presidency and a Supreme Court seat and, at 65, seemed a model of stability . . .
Later in life, Foxe earned a bachelor’s degree in communications from the University of Tampa in 1995, a master’s in marine science (in 2001) and a master’s in business administration (2004) from the University of South Florida, all magna cum laude. Coincidentally or nor, she also became a scuba-diving master at the University of South Florida.