I was out in my backyard, which is pretty high up and near the mountains. All around us we see various mountain ranges. I was waiting while my dog went out and to my left a flashing object in the sky caught my attention. We have many planes and helicopters fly over from time to time or that can be spotted in the sky, but this was unusual. I began watching it and noticed it appeared to hover over the mountain top. It was at about 15 degrees above the horizon and in the north northwest area. It was flashing white lights in what seemed like a circular motion and then occasionally another color or two would come in like a green/blue, maybe red. I know most airplanes have red and green lights, as do helicopters but the pattern and flashing of the lights wasn't typical of either. The object hovered above the mountain top for a few seconds and then I noticed it was beginning to descend. The object began moving at a very smooth pace. It began to disappear behind the frame of our gazebo and I moved to follow it the best I could. It continued to descend and eventually I lost it behind the mountain range. I haven't seen it since. What was unusual about it is I have seen plenty of planes and helicopters fly over those mountains but never disappear or hover above them. It could simply be some odd landing or occurrence but it definitely is unidentified. Right after it disappeared I waited to see if it would come back up, unfortunately it did not. I then came in and spoke to my Dad about it, who tried to take a very scientific approach to the issue. He went outside with me and I told him what I had seen and then re-enacted the event and even he said it didn't sound like anything he was familiar with. 1. I was in my backyard in South Corona, CA taking my dog outside. 2. I first noticed the object when I saw the bright and unusual flashing lights out of the corner of my eye. 3. When I first noticed it I thought it was probably an airplane that just happened to be at an odd angle with really bright lights. 4. It was moving at a very calculated pace and didn't seem to do any strange or odd movements except that it smoothly descended straight down and behind the mountain range toward the Chino Hills/Yorba Linda area, which I am not familiar with any aerial objects doing in this area. And there are plenty we have seen flying around.5. I felt intrigued. I am always looking up at the skies, day and night and its always interesting to see something out of the ordinary. I was excited to be quite honest but also disappointed I couldn't get a better view of it to know more of what it could be. It also disappointed me when it disappeared behind the mountain range. 6. I unfortunately lost sight of the object when it descended behind the mountain range.