Catastrophic mass killers from below and "Hypothesis of Dissociated Planet" 

Threats to Earth's living may be known beforehand if coming from above like asteroids and comets that, because observable from  long way off, might be among the types of catastrophic mass killers that Man in theory may successfully defend himself against. Its opposite may be those that come from below which may also be harder to detect and guard against.

The case of the 1908_A.D._(June30) explosion in Tunguska, Siberia because of certain observed effects strangely had been postulated by some  as explosion from underground (ref.#wh1).

It could be much, much more serious if underground explosion is of that kind that result due to dissociation of the planet's constituted mass, partially or else completely. This scenario as scientific fact may not really be an impossible reality, since planet constituents are only bound together by gravity which after all is, per unit mass, the weakest known force of physics in the universe (more in article at end titled "Anti-gravity effect & Astronomical mass dissociation" ).

A very large planet that apparently dissociated explosively between the orbits of planets Mars and Jupiter is the subject of a hypothesis that explain mathematical discrepancy noted in the distribution of planets in the solar system, as well as explain observed actual strange conditions on most of those planets, including moons and other mass bodies in the system. Originally the hypothesis was known by  something like "Exploded Planet Hypothesis", later by some othersimilar name, or by the "EPH" abbreviation (ref.#wh2 & ref.#wh3).
The space between Mars and Jupiter is a large anomalous gap for astronomers since the 1800's because of non-conformity to an expected spacing of planets as predicted by a so-called Bode's law (published in 1772).
The main asteroid belt orbiting the sun between Mars and Jupiter has total mass that is only a fraction of the mass for the planet that should exist in the space, as calculated by Canadian astronomer Michael Ovenden of the University of British Columbia. The mass of the missing planet as calculated should be as big as that of the giant planet Saturn (ref.#pe1).

The anomaly further may not exist indefinitely nor for long, as
indicated by a law developed by Ovenden in 1972 (ref.#pe1) showing that the configuration is unstable and that the major planets would reposition to more stable orbits if the solar system is subjected to forces like resistances to movements during passages through interstellar dust clouds (which do happen regularly; the resistances may also show up every 30_million years on the solar system's gallactic oscillation' crossings at our "Milky Way"s gallactic plane).

Comets that have been expelled by the explosively dissociated planet may be known by tracing, as was done, their orbits farther back in time, which may converge at a point between Mars and Jupiter around 4_million_B.C. Those that made first appearances were also calculated to complete their orbits in about 4_million years, an indication of the comets oscillating from point of origin, as well as an indicationof creation time (ref.#wh2, ref.#pe1).

Water may largely compose comets, and apparently was plentiful in the hypothesized exploded planet. Presence of water, an element so abundant on Earth, was considered factor in the explosion of that planet that apparently was dissociation of an extremely explosive kind, blowing debris that became comets to the solar system's outer regions as far away as the Oort cloud (ref.#pe1, ref.#wh2, ref.#wh3).