For many years the Podkamennaya Tunguska fall was neglected, partly because of the remoteness of the area in which it occurred, partly because of unsettled conditions in Russia; but chiefly because, in general, the Russian scientific and governmental officials simply did not believe the “fantastic” tales concerning the fall told by the native Tunguses, from which we have given a few details above. Belated study established, however, both the truthfulness of the Tunguse reports and the exceedingly unusual character of the meteorite fall itself. In spite of the overwhelming and, in fact, worldwide evidence that the Podkamennaya Tunguska fall was one of the greatest and most violent in history, no meteorites have ever been recovered from any part of the region devastated by its impact. It is the one and only true meteorite crater that is meteoriteless! This strange circumstance led the senior author to suggest, in 1941, that the almost incredible Podkamennaya Tunguska incident had resulted from the infall of a meteorite that, together with an equivalent mass of the earth-target, was transformed into energy upon contact with our planet. How can such extraordinary behavior be accounted for? [Illustration: LEONID A. KULIK PHOTO. SOVFOTO Infall of meteorite, June 30, 1908, had this effect on a Siberian forest. See p. 55.] The most obvious explanation involves a new and wider concept of matter. Ordinary terrestrial matter is regarded as composed of atoms having positively charged nuclei around which negatively charged electrons revolve. Suppose that the situation shown in the first diagram were reversed so that the nucleus of the atom were negatively charged and the charges of the particles revolving about it were positive, as in the second diagram. Matter built up from atoms like those in this diagram would bear somewhat the same relation to ordinary matter that -2 does to +2. Such matter is now known variously as _reversed matter_, _anti_-matter, or, as it was first called by V. Rojansky, _contraterrene_ matter. In recent years, scientists at the University of California Radiation Laboratory have produced experimentally all the fundamental particles necessary for the creation of contraterrene matter. What would happen now if a contraterrene meteorite penetrated into the ordinary matter of the earth? The answer is that just as an electron and a positron mutually annihilate each other when they collide, so the meteorite and an equal mass of the earth-target itself would vanish at the instant of impact. The nearest simple analogy to the actual complex physical situation is represented by the familiar equation -2 + 2 = 0.