My son's and I were observing a rare display of thunder and lightning here in Anchorage, and I was taking some video hoping to catch a cool streak of lighting and some thunder crashing and the like as we don't often get that sort of weather behavior in our part of Alaska. I noticed something out of my peripheral vision that seemed to climb rather quickly and seemingly close by. The shape was peculiar, and I would describe as oblong, perhaps rectangular and the color was dark/black. It ascended in a seemingly controlled manner, perhaps it was taking off, I'm not sure. I called my children's attention to it and after ducking under a short overhang on our building and using the corner of the building to steady my arms as I tried to get the object into frame. I noticed as it proceeded on its path across the city heading west from N/NE . It was in the general vicinity of both Merrill Field, a small light aircraft airport and JBER (Joint Base Elmendorf Richardson) Formerly Elmendorf AFB. Perhaps JBER or Merrill field has witness or radar confirmation as well. The object seemed to turn and tumble in a very peculiar fashion. It was quite larger than any balloon I've ever seen let loose and the way it moved and only ascended to a certain height seemed quite un-balloon like. I was able to follow it for about 3 minutes or so until it was blocked out by buildings and a tree in the distance. I have a cell phone video of the entire sighting, minus perhaps the first ascent.