This answer quotes from a book written by Jocelyn Bell Burnell who co-discovered the first pulsar in 1967 where the author mentions another astronomer who noticed a star was pulsing at around 30 Hz and recognized the frequency because
The Crab pulsar, with a period of 30Hz, is sufficiently fast that many people cannot see it; however some people can see 30Hz and Sue Simkin knows she is one of them – as a child she lived in an area where the main power supply was at 30Hz.
This page suggests that the area might be Detroit Michigan in the 1940's or 1950's.
But here I'm asking a more general question about mains frequencies well below 50 Hz.
Question: How low have mains frequencies gone? What areas once used 30 Hz, and why?