You could use the following term for territories that don't belong to anybody
Terra nullius (/ˈtɛrə.nʌˈlaɪəs/, plural terrae nullius) is a Latin
expression deriving from Roman law meaning "nobody's land",1 which
is used in international law to describe territory which has never
been subject to the sovereignty of any state, or over which any prior sovereign has expressly or implicitly relinquished sovereignty.
It applies to your example of the mid-ridge Atlantic or other geographical features.
"The Mid-Atlantic ridge doesn't belong to a country, so it is terra nullius"
Under the United Nations Convention on the Law of the Sea of 1982, the
international waters and international seabed are treated under the
common heritage of mankind principle by the signatories of the
if it's a piece of land above sea you can also simply say
You'd get thousand of hits on Google for it and it's the most commonly used term.
But for things owned by several countries, like North-America, then
is the right word to use, in its strictest sense.
of, concerning, or involving two or more nations or nationalities
For example "The Himalayas are an international mountain range"
See also The Helsinki Rules on the Uses of the Waters of International Rivers set up mainly to deal with the Nile situation.