Oxygen bubble

An oxygen bubble was an ability the Doctor's TARDIS possessed that enabled to extend oxygen around it's exterior shell so occupants outside the TARDIS could still breath even when in the vacuum of space. (TV: The Runaway Bride, The Stolen Earth, The Beast Below, Oxygen) This included being able to sit atop the TARDIS while in space. (TV: The Power of Three, The Power of the Doctor)

The oxygen bubble also extended several metres, allowing the Eleventh Doctor to breathe even though he was barely within the TARDIS' gravitational pull. (GAME: City of the Daleks) It could also prevent other particles from getting in, notably able to hold back water, even thousands of metres below sea level, even when the doors were open. (TV: Listen, Legend of the Sea Devils)