I haven't told anyone of this before, and I'm going to be deliberately vague on certain details even now, as I do not wish to be labeled 'one of those UFO kooks'. Since I'm no longer young, I do think I should at least put it in writing as there still remains the remote possibility of physical evidence.This event happened in May (or at least late spring/early summer) of 1957, when I was still a pre-teen farm boy. The time was mid-day, as my mother was preparing lunch; the weather was warm, winds were calm and the sky was CAVU. The geographical location was approximately 350 air miles north of Edmonton, Alberta. Exact co-ordinates would of course be provided to a professional investigator, if accompanied by the assurance of confidentiality.I was helping, or perhaps hindering, my father work on his 1954 Chevy pickup; he was cleaning it up in preparation for trading it off on a new car, which is one reason I remember the event so well. I was a pretty excited kid.The hood was up on the pickup, which was parked facing east and a mere 4 or 5 paces from the front of the house; he and my mother were carrying on a conversation through the open, south facing kitchen window as she prepared lunch.In those years there were no jet aircraft, or even air services of any kind (beyond the odd rumbling Beaver, Otter or Norseman bush plane) in that rather remote farming community. There was, however, at least one local resident who flew an Aeronca Champ, and another with a Piper J3.Later research indicates that there were, at that time, no established long distance commercial air routes that passed over that area using either piston engine or jet aircraft. There were no military air bases for at least several hundreds of miles, and there was no subsequent air or ground search activity. Under the circumstances, a search would have been impossible to conceal from the local population.In the north-eastern sky I noticed what I later came to know as a contrail, which I duly pointed out to my father. It was advancing in a north-to-south direction at considerable speed - as fast as I would now expect a modern fighter to 'pull a con', and somewhat faster than the contrails of current commercial airliners.It is important to note that at no time did I see "an object". I only saw the the effects of a flying object which remains unidentified.The trail began within my field of view, starting from nothing to a thin white thread and rapidly thickening to the now-common white contrail, but then quickly changing from white to light brown, then to denser, dark brown. The trail then abruptly turned from horizontal to apparently vertical. I acknowledge that the object may have been simply climbing steeply, either directly toward or away from my point of observation, giving the appearance of vertical flight. The 'trail' very rapidly continued darkening from brown to apparently black, whereupon there was a spherical 'puffball' as the object exploded. The 'puffball' would have perhaps covered a golf ball held at arms length. During this time, I do not recall any sound - this was simply a silent event in a brilliant blue sky. The event spanned an estimated 20-25 degrees horizontally, and an estimated 3-5 degrees vertically. The vertical element was brief, and took perhaps three seconds.I recall my mother asking what we were gawking at, and my father responding that we had just seen something explode. A few seconds later - I can only estimate 5 to 7 seconds, and I realise this is the most critical component of the observation - the blast wave arrived. It was strong enough to shake the open hood of my fathers pickup, and soundly rattle the windows in the house, which of course also brought my mother out to see what was going on. The remains of the contrail and the 'puffball' hung in the sky for at least several more seconds before gradually dissipating, giving her plenty of time for a good look. I am also aware that a blast wave travels faster than the speed of sound, and that the event probably happened a considerable distance away, which would have put it over a vast forested area that remains completely unpopulated to this day.During WWII, Canada had established a group called the GOC, or Ground Observer Corps. This consisted largely of Airforce veterans and private pilots; their purpose was to record the time, direction and type of aircraft that were noticed passing overhead. This was started, no doubt, from fears of enemy invasion but was continued post-war to aid in search and rescue efforts in a time when there was no reliable long distance radio contact with civilian aircraft. One such fellow (the Airforce veteran with the aforementioned Aeronca Champ, and one of the few people who actually had a telephone!) was a neighbor and family friend. My father immediately contacted him, and he in turn reported it as a possible accident. Several days later, he was simply informed that there were no aircraft reported missing.Over the next while, at least one other area farmer cautiously mentioned seeing the event and feeling the blast wave. Even then, I had the impression that it was spoken of warily; that if you spoke of something unexplainable, you were likely to be ridiculed.The event fascinated me enough that the details remain as clear as if they happened very recently; I truly don't think I've lost or altered anything. I have been at the exact point of observation regularily, and the angles, directions, and intervals remain clear in my mind.