Just as the name implies, solid wood flooring is solid hardwood, through and through. Unlike engineered flooring, solid flooring has no fabricated core. Because of this, it’s very easy to refinish, and almost never has to be replaced.
Solid wood reacts to changes in the surrounding temperature as well as to the amount of moisture in the air. Because of this, it often expands and contracts as a reaction to that. For this reason, you should only install solid wood flooring at or above ground level.