Earth mantle 'ocean'
Well, although it contains approximately the water volume of the Arctic Ocean, it is probably not exactly what you expect - not an ocean filling a cavity in the mantle like in Jules Verne's book, but rather just water dissolved in minerals (in concentrations ~0.1%, and 1000km deep). Although not quite well suitable for freediving, it is still quite a great discovery that will radically change our concepts about the mantle structure.
3-D seismic model of vast water reservoir revealed [English]
В недрах Земли обнаружен океан Жюля Верна? [Russian]
astro.cz: Obrovské zásoby vody hluboko pod povrchem Zemì [Czech]
Well, although it contains approximately the water volume of the Arctic Ocean, it is probably not exactly what you expect - not an ocean filling a cavity in the mantle like in Jules Verne's book, but rather just water dissolved in minerals (in concentrations ~0.1%, and 1000km deep). Although not quite well suitable for freediving, it is still quite a great discovery that will radically change our concepts about the mantle structure.
В недрах Земли обнаружен океан Жюля Верна? [Russian]
astro.cz: Obrovské zásoby vody hluboko pod povrchem Zemì [Czech]