Myself and a friend's sister were standing on a rear balcony at 18th and Franklin St. (SE corner, alley) when we both seemed to "sense" something. We looked up at the same time an observed a very large triangular craft or object moving slowly from north to south. The underside looked grey and the triangle tips were drakly colored. It had no lights or markings and made no sounds. It seemed to be cruising very low, perhaps only a few hundred feet above us.