Nighttime I was standing on a small second floor balcony overlooking the alley talking to a friend's sister. It was her apartment. Something, possibly a very low frequency hum, caused us to pause and look up in a slight northerly direction. The balcony was on the east end of the building which was a 2 story structure. I saw clearly from street and city lights a very large triangular flying craft moving very slowly toward us. It went overhead traveling south and disappeared over the tree tops. It made no sound that was noticeable unless it was very low decibel level. It was a quiet night and I do not recall any traffic which would come in waves when it did as Denver had street lights that were synced so traveling at speed limit one would hit all green lights. Time of night was a factor in the quiet. The craft had no lights, no visible windows, no markings and seemed close enough one could hit it with a thrown rock. After it disappeared from view the woman and I just looked at each other with stunned surprise and unspoken "did you see that?" eyes. We went inside and told the other 2 inside what had happened. They seemed only a little interested. I was sober but may have smoked marijuana at some point prior as was our habit to do. The marijuana was not powerful in those days unless one spent extra money for more potent product which would be rare for us considering our financial situations. We rarely drank alcohol or imbibed in any other mind altering substances for that matter. The whole incident was about 15 to 20 seconds in length. Probably was close to 11 PM. It was quite remarkable to me immediately that what I was seeing was flying so slow and was quite large and virtually silent. No engine sounds and no air friction sounds and I would estimate it traveled at about 20 MPH. I looked for any marks such as numbers, letters, doors windows or lights and it had none of these on the underside and had no visible sides. It appeared to be fairly flat from my angle of view. It was predominately (90%) dark or drab green and the triangle tips were rounded not pointed and were a dark drab burgundy color. It was low enough to be illuminated by mercury vapor street lights. I never at any time thought it to be anything of human or earthly origin as flight must have a sound and aircraft must have lights. Also it was flying too low within a mile of downtown Denver. Never heard any reported sightings of it by others. Any mention to this day is of little interest to anyone. I do not have much interest in alien/UFO stuff or sci/fi. I am more reality based person and find "mystical" beliefs to be mostly nonsense. I never believed Roswell was a UFO crash site or that aliens were found. I do not believe mind-reading or psychic abilities exist. Most mysterious occurrences have explanations. I believe it is possible other beings exist somewhere but why they want to come here I find to be odd considering how much it would take to travel that far. Humans would have to be to them what ants are to us. The scale of size of human existence though is oddly similar as per reported alien sightings but abilities seem to be quite dis-similar. Memory plays tricks and eye to brain communication is not as exact as most think. A magician can astound people although they know full well that there is no magic involved. I personally knew a magician name of Steve Aldrich. Amazing magician with jaw dropping tricks but not real magic. I can make a coin "disappear" in a bandana... I do not know if it is related or not but it was in this time period that I started experiencing debilitating anxiety attacks and still do to this day. The woman, I was learn years later, had been institutionalized with schizophrenia at some point. She was not helped by medications and had very little lucid thought and acted childlike. I had known her to be responsible, independent and sometimes pleasant but quick to anger which could be off-putting. I had no interest in her other than she was a friend's sister and she had let us stay from time to time at the apartment if we needed. The friend and I were typical long-haired guys with acoustic guitars, not uncommon in those days and that area ("Capital Hill") She was employed as a maid in a large Denver hotel (The Raddison I believe) I worked for a macrame/weaving supply business called Strings and Things. I was supervisor of the jute and wool dyeing department which was small and only had 3 or 4 part-time employees. It was a remarkable event that somehow nothing could be done or said and was rarely if ever recalled by me until I became a member of the computer amd Internet age. I did some research on the 'Net and saw a number of similar episodes but they all seemed to be later in time and the triangle was the only common theme. Others all had lights and sound plus were far off sightings.