It was only explored in a gross sense. As a measure of this, it's estimated that we only know of between 2/5ths and 1/15th of all species (
http://www.enviroliteracy.org/article.php/58.html). The oceans, which comprise 95-99% of the living space on the planet by volume (Bill Bryson, A Short History of Nearly Everything), are almost completely unknown. There are still places to go to get away from things.