Looked flat, cruising at low altitude on horizon and appeared to drop a triangle structure.

What I saw looked like a long flat thing cruising at a pretty low altitude just above the western hills as I rounded the curve on interstate 91 north in the West Springfield area. I don't know whether it was West Springfield or Holyoke. I also don't know how many witnesses but the highway is pretty busy at that time so there were probably others who saw it. It was moving surprisingly slowly and was also strange in that it appeared flat. Suddenly something triangular dropped beneath it and appeared to fall from it towards the ground. It also looked thin and slight. I wish I'd had a a dash cam! And I've never reported anything like this before because I tend to be very skeptical of these reports. I make no speculations about what I saw.