Underground downspout drain line
I have one downspout that dumps into an underground 5" flexible drain. With very heavy rains, water gushes out of the top of the tubing, around the 4x2" aluminum downspout. I removed the aluminum downspout and found nothing within reach blocking the tubing. I ran an electrician snake and it went about 6' into the drain line, and stopped abruptly. I cannot tell where the drain line goes, but I assumed it might end many feet away among some bushes and a tree. I wanted to find its end so I could verify it was widely open. I ran water from a garden hose in it for 15 minutes to see if water appeared downhill in the yard. It did not overflow, and I could not find any wet spots anywhere. The line goes under a concrete walk, so digging it up is not feasible. How do these usually terminate, and how does one track its course? It does not empty into any municipal storm system, because there is none in the area. It may have taken a turn that my snake could not negotiate, and still might be longer than I could traverse. I would like to see if I can do anything to improve its drain rate. What else can be done?