No. 21: May-Jun 1982
In SF#20, concentric rings of gravity anomalies centered in Canada are described. A similar patter of magnetic rings has shown up in the Yucatan peninsula. The inner ring is 60 kilometers across; the second, 180 kilometers. The rocks causing the magnetic anomalies are about 1100 feet down. Since these rocks are probably Late Cretaceous in age, this potential impact feature may be the eagerly sought scar of the asteroid impact that some think wiped out the dinosaurs and left an iridium-rich layer all over the world.
(Anonymous; "Possible Yucatan Impact Basin," Sky and Telescope, 63:249, 1982.)