Of Circles and Erkels

The strength of an arch is not only found
In a half circle structure over your head
It is also found deep underground
It even occurs deep under seabed.

For an arch is one of those configurations
In a circular tube in a deep long tunnel
Its strength is used to join two separate nations
Like England and France are joined by the Chunnel.

Compression still works in a completed circle
It’s pressed on itself all the way round
You needn’t be a nerd or an Erkel
To know that the arch is the strongest found.