to that study, the most frightening object is a giant long-period comet with a warning time of a few years only and therefore no time for any intervention in space or settlement on the Moon or Mars. The probability of a giant comet impact in the next hundred years is 2.2E-12. Individual vs. species risks Although existential risks are less manageable by individuals than – for example – health risks, according to Ken Olum, Joshua Knobe, and Alexander Vilenkin, the possibility of human extinction does have practical implications. For instance, if the "universal" Doomsday argument is accepted it changes the most likely source of disasters, and hence the most efficient means of preventing them. They write: "... you should be more concerned that a large number of asteroids have not yet been detected than about the particular orbit of each one. You should not worry especially about the chance that some specific nearby star will become a supernova, but more about the chance that supernovas are more deadly to nearby life than we believe." Difficulty Some scholars argue that certain scenarios such as global thermonuclear war would have difficulty eradicating every last settlement on Earth. Physicist Willard Wells points out that any credible extinction scenario would have to reach into a diverse set of areas, including the underground subways of major cities, the mountains of Tibet, the remotest islands of the South Pacific, and even to McMurdo Station in Antarctica, which has contingency plans and supplies for long isolation. In addition, elaborate bunkers exist for government leaders to occupy during a nuclear war. The existence of nuclear submarines, which can stay hundreds of meters deep in the ocean for potentially years at a time, should also be considered. Any number of events could lead to a massive loss of human life, but if the last few (see minimum viable population) most resilient humans are unlikely to also die off, then that particular human extinction scenario may not seem credible. Ethics Value of human life "Existential risks" are risks that threaten the entire future of humanity, whether by causing human extinction or by otherwise permanently crippling human progress. Multiple scholars have argued based on the size of the "cosmic endowment" that because of the inconceivably large number of potential future lives that are at stake, even small reductions of existential risk have great value. In one of the earliest discussions of