In 1960, a manned bathyscape reached the bottom of the Challenger deep, 35,813 feet.
Whilst a bathyscaphe is not a "submarine" in the normal sense of the word, it is nevertheless a manned, free diving, self propelled and dirigible vessel.
4 years before the structure was discovered, people had made controlled descents to 2.65 times the depth that the structure was found at.
Bathyscaphes have windows. It is easy to think of a number of possible arrangements of hooks, arms magnets and the like that could be used to position a structure.
The bathyscaphe works on the basis of a buoyant an incompressible float filled with gasoline or some similar oil, offset by ballast that can be dropped when required. Alternatively, instead of dropping ballast, you can let the gasoline out and let water into the float. An "underwater balloon" made on the same principle could have been used to lower the structure in a controlled way, guided by a bathyscaphe.
Putting the structure there before 1964 would have been a big undertaking but certainly not impossible.
Why it was put there is another question.
I have learned something today. For nearly 50 years, I have thought it was bathyscape, not bathyscaphe.