It’s expensive! And if oil is ever expensive enough to justify the cost of drilling in Challenger Deep then by golly you start investing in alternative power instead. Semi-submersible deep-water drilling platforms. Drillships. We drill kinda deep in the ocean but like, not actually deep. Record drill depths and the vast gulf from there to the Mariana Trench.
Gravity meets Aliens. Pressures and temperatures that are more deadly than getting spaced out of an airlock. The deepest hole humans have ever drilled was barely deeper than Challenger Deep is under the surface of the ocean.
Nitrogen narcosis: it’s bad for you! Atmospheric mixture issues under pressure. Sealab I, II, and III. Aquanatical research. Sperm whales get the bends. Oh no your suit some kind of exploded. Monsters of the deep. Deep sea gigantism.
Engineering against 16,000 psi and freezing temperatures. Concrete strengths and saltwater issues.