Dirt floor

For a nice hard smooth floor...of dirt - tamp it down with a heavy object, then rub it with something acidic, like citri-solve.

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From The $50 and Up Underground Housing Book by Mike Oehler (link is to a 115 page pdf file) -

"Some folks in the southwest

sweep linseed oil into their floors, wait a day or so, and repeat the process. Though still allowing "give" this sets the earth nearly like concrete; you can wipe up a dropped egg

without getting any earth on the rag."