This is the 10110th most frequent Russian word.
батискафом
A "батискафом" is the instrumental case of "батискаф," meaning "bathyscaphe," a deep-sea submersible vessel.
In this sentence, 'батискафом' is used as an instrumental case noun representing the tool or vehicle the subject used for exploration.
Вчера я исследовал подводный мир с батискафом.
Yesterday, I explored the underwater world with a bathyscaphe.
Here, 'батискафом' is used in the instrumental case to describe what allows or enables the action.
Батискафом можно погрузиться на большую глубину.
With a bathyscaphe, one can dive to great depths.
In this sentence, 'батискафом' is part of a prepositional phrase indicating the instrument involved in the rescue story.