The same would be true of the passage if we plumbed the middle.

When we came through it the ship scraped bottom time and again.
As a matter of fact,--the way I figure it out,--she was simply bumping against the upper edges of a crevice that reaches down God knows how far.

We took no soundings, you will remember, until we swung out into this pool.

I'll bet my head that that cut through the hill yonder is a mile deep.

Earthquake fissures seldom go deeper than that, I've heard.
Generally they are mere surface cracks, a hundred feet deep at the outside.