The Pacific Ocean's Mariana trench is so deep, it could accomodate the entirety of Mt. Everest with thousands of meters to spare. Now, the most precise measurements of the trench to date indicate that it's even deeper than we once thought.
The new survey pegs the depth of Challenger Deep (the deepest known part of the trench) at just shy of 11,000 meters — that's about 75 meters greater than previous estimates.
Of course, we've already made several descents into Challenger Deep, so this adjustment is more about being right than it is about calling attention to uncharted territory — as far as I know, there's nothing about this that implies there's an extra 75 meters of depth that we've yet to explore.
Unless, of course, there are 75-meter-deep fissures suddenly gaping into existence at the bottom of the ocean, in which case we should probably figure out what the hell is going on.