The hole got bigger and more complicated

Well that 5th fence post hole where I ran into the drainage(?) pipe got a little bit bigger this weekend. I made the executive decision to take my fence about a foot closer to the property line in order to sidestep the pipe. The ghosts of builders past came back to haunt me though. Just a few inches away was a big concrete mass in the dirt near the same depth with another drainage pipe leading to the surface. I take it to be a downspout from a previous building (which I learned from an old insurance map used to have its corner almost exactly at that spot). As I dug farther down the length of the pipe, I saw that the two connected with a Y pipe that had been cracked. See the picture for this great work of pipesmanship.

The concrete blog at the top and slightly right is where the downspout came down. There is a small piece of cracked pipe stuck in it leading to the surface which is outside of the picture. The two pipes Y together and then exit left. The pipe from the bottom right to the top left may still be active in carrying water though, I'm not sure.

I pulled the cap on the side of the downspout at my building and poured water down it to see if water was flowing through the pipe. I didn't see any, but it just might not have been enough water, so I went and checked all the downhill sewers to see if the water made it to them some other way. All dry. Now I'm confused on how (hopefully not if) my downspouts get to the sewers....

At this point I'm thinking that I'll dig all the way around the concrete mass from the old downspout, remove it, and then use concrete to semi-patch the other pipe before burying it again and building my fence post alongside. Just in case its still active somehow. Hopefully this concrete mass/old downspout doesn't have anymore surprises so I can focus on the fence.