Ever seen a ferrofluid, which follows the shape of magnetic fields? Same thing, but with gravity.

Of course, that only accounts for a fraction of those 16 meters... but there's a lot of ocean water. Get it moving (because the Moon and the Sun move, and the Earth rotates under them, and there's a whole lot of ocean currents on top of that, due to differences in water temperatures and salinity, and coriolis forces, and winds, and whatnot) and it builds up a lot of inertia.

Push it into geography that keeps narrowing and narrowing like a funnel, and the only place it can go is in, and up.

Water gets in there, wants to get out, but there's a whole damn ocean pushing it in, so it has no option but to keep accumulating into the funnel.

Also, having the geography look a bit like a Tesla valve that'll easily let water in but not so easily let it out probably doesn't help either; place's bound to get close to overflowing, before it can empty itself out.