The Desert Fallout Hypothesis proposes that Earth's major deserts are not the slow product of climate drift alone, but resonance scars of sudden cosmic events — specifically, atmospheric airbursts of alkali- and silica-bearing meteoritic bodies during the Younger Dryas onset (~12.8 ka) that rapidly sterilized soils across multiple continents simultaneously. The Law of Selective Desertification identifies mountain barrier geometry as the decisive variable: the same global fallout event trapped sterility permanently in Australia's caldera-like interior, shadowed the Taklamakan and Gobi behind the Himalayas, scarred the Caspian lowlands through partial shielding, and split North America into desert southwest and fertile midwest along its mountain chains. Silica phases and exotic microspherules serve as the primary durable tracers, with closed basins like the Dead Sea functioning as ion traps preserving the sodium record that open soils lost millennia ago. The deserts we see today are not random — they align with fallout pathways and the geometry of mountains.