Though digging a hole connecting inside and outside of a prison is a major way to escape from the prison, I've hardly heard that prisons take measures for preventing inmates from getting away from the prisons via dug tunnels.
I guess it will be effective to build thick concrete plates under walls' ground. But, the problem is that it won't be effective in the case that the tunnel is deeper than the concrete plate. So, the concrete plates should be longer as long as authorities admit.
Diary of a rail fan in Kansai,Japan(Ver.2)
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