Many alien worlds will most likely be water worlds. I was wondering what the deepest possible global ocean dept is where land is still possible. I think that volcanic islands will still have a chance to form even after continents are no longer possible since they can have quite steep slopes which are supported by water. I'm aware that this isn't anything we can describe easily, so all I'm looking for is a rough estimate. Would there be any reason not to scale this estimate linearly with gravity to adapt it for worlds with nonterran gravity?

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    $\begingroup$ Depends on how high you want to make the mountains, really. $\endgroup$
    – Halfthawed
    Commented May 6, 2020 at 19:42
  • $\begingroup$ Related: worldbuilding.stackexchange.com/q/30513/627 $\endgroup$
    – HDE 226868
    Commented May 6, 2020 at 20:22
  • $\begingroup$ @Halfthawed Are mountains the determining factor? If there is no continental crust, only oceanic curst is available for mountain building. The result of that is significantly less impressive. I think vulcanos can get higher underwater than any mountain range. $\endgroup$ Commented May 6, 2020 at 20:46
  • $\begingroup$ Perhaps. But water can always be added to planet after a mountain range is established via comets, so I wouldn't be too worried about planet formation. $\endgroup$
    – Halfthawed
    Commented May 6, 2020 at 21:13
  • $\begingroup$ @Halfthawed You are casually suggesting adding more than the current oceans worth of water late during the development of a planet. That's an event akin or possibly exeeding the late heavy bombardment. I'm not sure that a mountain range or even continents would survive that. $\endgroup$ Commented May 7, 2020 at 6:11

1 Answer 1


On Earth, the deepest point in the ocean is Challenger Deep (36,200 feet) and the tallest mountain is Mount Everest (29,029 feet). Mount Everest is roughly 65,000 feet taller than Challenger Deep, so if you parked the two next to each other and raised the ocean levels to just below Everest's peak (RIP life) you could have an ocean more than 12 miles deep. There's no hard and fast reason that the difference couldn't be even greater in your fictional world.

  • $\begingroup$ I think the OP's asking how great the distance could possibly be, though. $\endgroup$
    – Halfthawed
    Commented May 6, 2020 at 20:11
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    $\begingroup$ Everest sits on a continental plate, though. I there is too much water, continental pates will never form. $\endgroup$ Commented May 6, 2020 at 20:43

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