The answer is
A: A Vast and Limitless Space
The abyss , you can think of it like space. Space is unlimited, from the theories and hypothesis. The scientist in these two sentences doesn't know how deep the abyss is, no length width or hight. If you've watched Gravity Falls, there's one episode where Gruncle Stan has 'the neverending hole'. The children didn't believe it was a 'never ending hole,' until they fell in, they assumed they'd be falling forever.