The other day I went out by myself on a little 18 mile jaunt. I wanted to push up a fairly steep hill on this particular route, and prior to that steep uphill, there is about a 2 mile downhill - windy road, levels out in spots, partly shaded country road - speeds between 18 mph and 35 mph.
So it's a nice little downhill road, and to make it even nicer, part of it was freshly paved - not chip seal, but actually paved blacktop. Sweet! They must have repaved it very recently.
So I'm cooking along when all of a sudden - BAM! my right hand comes off the hood and I'm in a world of "this isn't going to end well". I hit a manhole cover that, due to the road being new paved, was even deeper than before.

Thank God, I was able to maintain control and continue on. There was another manhole down the road a bit that wasn't as much of a problem.

I went back a couple of days later in my car to, um, "mark" the area so that I and others might not die on future rides.
Even as I slowly drove up that road, I was only able to spot one manhole. So I turned around to go back down to find that other elusive manhole. That's how difficult it was to see these things. I actually had my head out of my car window trying to spot these things and missed one the first pass.

So just a PSA - as much as you might like downhills, be careful. I'm sure I'm preaching to the choir for the most part. But be extra careful in shaded and partly shaded areas.