I am unsure on what your questions mean, here. Submarines adjust their density by filling and emptying tanks. When a sub fills enough tanks, it dives. As long as the sub has the structural strength to survive the depths that it is diving to, the sub can get to any depth.

There is offshore drilling for oil - those might be the largest depths to which we have gone under the ocean.

How deep do Offshore Rigs drill? Depending on the rig type, offshore rigs are rated to drill in water depths as shallow as 80 feet to as great as 12,000 feet. The greatest water depth, a jackup, can drill in is 550 feet, and many newer units have a rated drilling depth of 35,000 feet.

The dinosaur extinction might be the biggest event that involved mass falling into the ocean - dinosaur extinction.

I'll let you research the Rupert's drops - the ones I see here are only small ones - Prince Rupert's drops are toughened glass beads created by dripping molten glass into cold water, which causes it to solidify into a tadpole-shaped droplet with a long, thin tail. Wikipedia.

Here's an interesting aspect of your question: life in sea floor.