Leavitt himself suggested the analogy of the upper atmosphere and the depths of the sea as equally inhospitable environments, but equally viable. In the deepest, blackest regions of the oceans, where oxygenation was poor, and where light never reached, life forms were known to exist in abundance. Why not also in the far reaches of the atmosphere? True, oxygen was scarce. True, food hardly existed. But if creatures could live miles beneath the surface, why could they not also live five miles above it?
