According to conspiracy theories, aliens would hide in secret underground bases jointly operated by our world governements and the visitors. Here is a list of the reported alleged locations of the D.U.M.B.S.
With 2 underground bases being built per year in the US at the moment. The average depth of the bases are four and a quarter miles underground.
The bases are on average the size a medium sized city. Each DUMB base costs between 17 and 26 billion dollars to build which is funded by MI6/CIA drug money.
Each underground base employs 10000 to 18000 workers.
A nuclear powered drill it used to dig underground. This drill goes through rock at a tremendous rate and literally melts the rock away to form a smooth glass like surface around the edges of the tunnels.