I was camping in the Lost Creek Wilderness area near the Rolling Creek Trail on the north-western part of the valley near Wellington Lake (appr. 39.335346, -105.407524), appr. 2.5 miles from the lake.
At about 10:30pm on Friday June 21, with my son (25 yrs old) and his girlfriend (also 25), we heard a jet flying a normal approach to the Denver Airport from the south to the northeast. While peering through the forest canopy in the direction of the jet, I was able to see the flashing lights of the jet at normal altitude high in the sky. Shortly after first seeing the jet's flashing lights moving away in the distance, I saw a vivid, bright green-white, solid light flying through the trees but in front of the "Castle", which is a large rock formation at the south side of the valley. The light was clearly at very low altitude, and was in fact within the confines of the valley, lower than the ridges that form the perimeter. There was still enough ambient light to see the horizon of the ridges to the south/southeast, and the light was lower than the ridges. It was in a slightly descending attitude, and I could see it clearly through an opening in the trees, and then intermittently through the tree-cover as it traveled to the north/northeast. I lost sight of it after about 5 seconds. It appeared to be traveling at about 50 to 70 mph possibly. There as no sound whatsoever from the object. It did appear that it could have been a small plane or helicopter, but the area is remote, and other than the faint sound of the other jet flying away from our position, there was no engine or other sound from the low altitude light. Highly unlikely that a seaplane could have been attempting a landing on Wellington Lake, as it is very small, and the sound would certainly have been very loud. The night was very quiet, and the sounds of campers at Wellington Lake were audible in the distance from miles away at the time. My son did catch a glimpse of the object as it passed behind the trees.
We thought it could have possibly been a drone or some sort of remote-controlled object, but the location and time of day made this seem unlikely. Also the light was quite bright, similar to the headlight of a car. There is no possibility that this was a land vehicle. The light was viewed with the "Castle" as backdrop, which is a sheer cliff around the north sides. There were certainly people camping at the Wellington Lake campsites, and someone there may have also seen this object clearly, as it's flight path appeared to be towards the lake.