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. At the same time that the pilots had their encounter, the Coimbra Meteorological Observatory registered extraordinary localized variations in the earth's magnetic field, as proven by charts at that establishment.