tim birr wrote:
My house is about 50 years old. All downspouts from the gutters go
into underground piping and exit "downhill" from a four-inch black
plastic pipe into a drainage ditch.

Near the end of last winter, noticed that water was pooling up from
"underground" near one of the downspouts.

Being lazy and clueless, ignored the problem. But as rain season
approaches, figured I better fix this -- we get about five feet of
rain per year.

I will dig out and see what is going on (I suspect a hole
underground). I will patch if I can, but if I need to run a new line,
any advice?

You could try using a hose with and without a blaster nozzle to try to clear the
pipe. I've had good results with this in the past, and since your drains go to
an open outlet, your chances are better.