8:06 p.m. local. A flight of four F-84 Thunderjets took off from Ota Air Base, Portugal, on a routine practice navigation mission. After the pilots reached Granada and started a port turn to change course to Portalegre they noticed on the left and above the horizon a very unusual source of light. All of a sudden the thing grew very rapidly, assuming five or six times its initial volume, becoming quite a spectacle to see . . . [then] fast as it had grown, [it] decided to shrink, almost disappearing on the horizon, becoming a just visible, small, yellow point. These expansions and contractions happened several times, but without becoming periodic and always having a pause, longer or shorter, before modifying volume. At 2038 hours the lead pilot decided to abandon the mission and to make a port turn in the general direction of Coruche since nobody was paying any attention to the exercise. They turned about 50° to port but still the thing maintained its position of 90° to our their left which could not be possible with a stationary object. As they were near Coruche the "big thing" suddenly and very rapidly made what looked like a dive, followed up by a climb in their direction. Then everybody went wild and almost broke formation in the process of crossing over and ahead of the UFO.