Lets gather what we know so far

Apparently Godzilla has attacked at least 1 times and also godzilla or another monster monster caused a nuclear incident 15 years ago in which Bryan Cranston's character lost his wife and caused a japanese city to be abandoned and this was all kept in secret by who i think is M.U.T.O/MONARCH.

Also there are some top secret documents to a nuclear contaminated island near San Francisco that has served as a nuclear waste disposal from 1946 and many of them occurred before 1960