On April 15, 2015 I was traveling in my car with my boyfriend in Williamson County, TN near the Cool Springs area. After turning from Moores Lane onto Concord Rd traveling N/NW we both observed a gradient grey (dark grey to light grey) triangular craft hovering very low over the tree line. At first we thought it might be an airplane but both noticed it wasn't actually going anywhere. The craft had a light/lights at each corner that were bright, didn't pulsate or blink, but appeared to change from white to yellowish to a bluish color. The craft appeared to be stationary for 20-30 seconds over the trees, not traveling at an angle and wasn't moving in any direction. Suddenly, the craft shot at about a 45 degree angle up and to the left (one straight line). The second before it moved, the whole craft flipped onto its side and the lights were facing us. It traveled the length of two football fields in the blink of an eye. It hovered at its new location for about 10 seconds and disappeared. While this was happening we were both watching it but not really talking about what we were seeing. We were both calm, didn't feel alarmed, but I remember thinking that a plane that close to the ground in a residential area was dangerous and wondering if the storm rolling in made it fly lower. It was until it flipped from perpendicular to the ground to sideways perpendicular to US and shot upward so fast that he and I questioned each other as to what we were looking at. We still were calm but somewhat nervously laughing that "um, exactly WHAT did that thing just do and did you see that?!" By the time we asked each other it was gone.The most pronounced fact about this is how clearly we could see it. I could likely draw it. It was not an airplane. The gradience of the color was....odd. And the speed with which it traveled, flipped and then STOPPED in mid air defied anything we had ever seen and the lights quite bright. There was a large and rather violent storm moving into the area and the craft was not IN the storm but far enough between us and the storm line (storm was at least 4 miles away in the Nolensville Rd/Lenox Village area) where the clouds provided a perfect backdrop to see it. It was too close to us to have flown into any cloud cover because the cloud line consisted of large cumulonimbus off into the distance and clear skies where the craft was. We had a clear view of the sky and did not see the direction in which it disappeared. We didn't lose sight of it in the trees or at any time...it just vanished. We drove around looking for it but couldn't locate it after it disappeared. It was never higher than a 50-75 story building. Afterward we didn't even really talk about it...just went about our way, which now that I write this, seems very odd.