Is it true that an island that isn't on the map that the country nearest to it, then it's theirs

Ive been told that if someone finds an island that is not on the map then the country nearest to it owns it. Is it true? What about the person that found it? Can't they keep it for free

if you find unclaimed land, you can stick a flag in it and call it your own.

the sad situation is that there is very very little unclaimed land in the world.
since satelites we kinda know every bit of land there is.

antartica was even devided over several nations. eventhough they dont inhabit it. however, the united states doesnt recognise that treaty. which leave the US share unclaimed. go stick a flag in it!

That is irrelevant.
We have had satellites orbiting the Earth for the last half century, taking picture of every square inch of the planet, and there is no such thing as an island that is not on the map.

Because the presence of natural resources on and below the sea bed, and the exclusive economic zone that extends for one or two hundred miles around every shore, any land and island had already been claimed long ago.

Think about it.... What happened hundreds of years ago in the Age of Discovery when the explorers like Columbus, Magellan and de Gama discovered islands? They claimed them for their country, no matter how far that island was from their home.
You could claim ownership of it for your country, and you could own the land for free assuming no one else already lives there. (It's not like hundreds of years ago when non-Europeans were considered non-humans.)
Just don't worry about it too much. It's highly unlikely than any land worth finding is, as yet, undiscovered.

No, that is not true. The island belongs to the country that is willing to use the most military power to keep it. That has always been true, and will continue to be true.