Black hole formation in the atmosphere by cosmic rays is a very interesting possibility. If experimentally verified, this would have a tremendous impact on physics as it would provide a conclusive proof of the existence of extra dimensions. If large extra dimensions exist, gravity becomes very strong at the collisional energy scale of the highest-energy cosmic rays with air nuclei.

In this theoretical model, the carriers of the gravitational force (gravitons) are able to propagate in more than three spatial dimensions, unlike ordinary particles. Thus gravity appears to be weak only because it is diluted in a higher-dimensional spacetime that other particles cannot see. This increased force of gravity is the key ingredient to the formation of the BH. Ultrahigh-energy cosmic ray protons are generated from various extra galactic sources (such as supermassive black holes, active galactic nuclei, etc). They have a tremendous amount of energy (cosmic rays with energy above 1020 eV have been observed). These protons interact with cosmic microwave background photons to produce ultrahigh-energy neutrinos, called cosmogenic neutrinos. These cosmogenic neutrinos, interacting with nucleons in the atmosphere, may produce miniature black holes; for sufficiently small impact parameters the colliding particles may "enter" a spacetime with more than three spatial dimensions, where gravity is no longer weak a force.

These miniature black holes are extremely hot and quickly disappear by emitting radiation (Hawking evaporation). Their average lifetime is of the order of 10-25 secs. This is the reason why these black holes don't swallow us, contrary to popular belief: They don't have the time! What do they decay into? A bunch of particles that further interact with air nuclei to produce a host of other particles. This "cascade" is what we call an air shower. The figure to the left shows the development of fifty of these showers for black holes (black curves) and standard particle events (red curves). The plots gives the number of electron pairs (y-axis) as function of the so called slant depth, which measures the distance in the atmosphere from the point where the black hole is formed (x-axis). The left side of the plot corresponds to high altitudes and the right side to the ground level. The light observed by cosmic ray telescopes is produced by the interaction of the electrons in the air (fluorescence light). Therefore, the plot describes the amount of light that is observed in an air shower as a function of the altitude. It can be seen that the air shower light grows in intensity, reaches a maximum and fades until no light can be detected.

Cosmic ray observatories (e.g. the Pierre Auger Observatory in Argentina) are in principle able to distinguish black hole air showers from ordinary air showers. Hopefully, someday we would be able to detect atmospheric black holes and probe the existence of extra dimensions.

To Run your Own Shower!