abyssal plains
... that despite covering more than 50% of this planet's surface, abyssal plains are among the least explored regions on Earth?
... that despite covering more than 50% of this planet's surface, <b id="mwBkM"><a href="./Abyssal_plain" id="mwBkQ" rel="mw:WikiLink" title="Abyssal plain">abyssal plains</a></b> are among the least explored regions on Earth?