Thought it was an airplane but wasn't

This sighting happened at least twenty years ago but I will never forget it. It was a warm sunny day in July about four o’clock in the afternoon when I had noticed something in the sky approximately a mile away to the north, moving west about fifty degrees above ground level, (ninety degrees being straight up) moving slowly. It was about the size of a passenger plane such as a Boing 747 and was cigar shaped with what looked like having a silver skin. There was absolutely no sound, but I thought it was because of its distance from me. At first, I thought it was an airplane but then I realized I couldn’t see a rudder or wings. Still thinking it might be an airplane it started getting brighter as if the sun was reflecting from it which made me believe it was turning northerly. To my surprise, there was a vertical hole in the sky slowly opening about an airplane’s length ahead of the flying object. The closer the flying object got to the hole the larger the hole became with a brilliant light shining thru reflecting off it. It slowly passed thru the hole and once thru, the hole slowly closed behind it. From the time the hole starting opening, the object passing thru, and the hole closing, took about ten seconds. I didn’t have a reference to gage distance between it and myself so distance from me is best guess. The hole looked like a portal for the object to pass thru to the unknown.