Traffic tends to pound them into the frame, and rust and dirt glue them in as well. Sometimes the city welds them on temporarily, if the Prez or someone is coming. Gas explosions as well as hydraulic pressure can blow them out sometimes.
Related fun fact: Monte Carlo GP on this weekend!
NB
This week’s Mythbusters tried to suck a manhole cover off using an IndyCar. They succeeded in small scale in a NASA lab water tunnel, but mostly failed in full scale. But for safety reasons they probably didn’t push the extremes in car speeds. However, an “antique” IndyCar or F1 car from the “ground effects” era would probably be the best vehicle to test the myth on. Those cars were banned because if the car’s downforce aerodynamics were compromised by a part failure or damage from an accident, there was little hope for the driver to control it.
Related other fun fact: The F1 Monaco Grand Prix is run on the streets of Monte Carlo, so it’s not “like Monaco and Monte Carlo”, it’s more like the same place.
My first thought was the previously mentioned Bernoulli effect creating a low pressure zone above the manhole combined with wind being scooped into the tunnels from multiple storm drains increasing the pressure under it might pop the cover with a bit of luck tossed in.
That would be my first thought, too. Houston occasionally floods, usually due to slow-moving tropical systems. The water goes down quickly, but you see that a few manhole covers have just popped off.
One reason it’s not smart to wade through flood water “for fun.” Then there’s the sewage. And the water moccasins…
I thought the same as well. Back when Allison hit I was at a stoplight trying to get home and saw the manhole cover in the middle of the intersection pop off and fill the street with rapidly rising water. I had to run the light because my car would have been flooded if I’d waited for it to change.