Unidentified flying objects.

Confrontations Vallee, Jacques 1990 Flying saucers Menzel, Donald H. 1953 Gods & spacemen throughout history Drake, W. Raymond 1975 Guardians of the universe? Story, Ronald 1980 Our haunted planet Keel, John A. 1971 Selected by extraterrestrials Tompkins, William Mills 2010 The bermuda triangle mystery-solved Kusche, lawrence david 1981 The Ufonauts Holzer, Hans 1976 UFOs and IFOs Soule, Gardner 1967 Unacknowledged Greer, Steven M. 2017