Here is a map of the world marking the endorheic basins: https://en.wikipedia.org/wiki/Endorheic_basin#/media/File:Ocean_drainage.png https://en.wikipedia.org/

Here is a map of the world marking the endorheic basins: https://en.wikipedia.org/wiki/Endorheic_basin#/media/File:Ocean_drainage.png https://en.wikipedia.org/wiki/Endorheic_basin#/media/File:Oc... Everything in gray is an area where simply following the current gradient of the land downwards will not lead you to the ocean. As you can see, this is a sizable chunk of the world.