system
Given the following post, respond as if you're commenting on Reddit:
human
When the first nuclear submarine, the Nautilus passed under the North pole in 1958, how did they know there is enough room for it between the sea floor and the bottom of the ice?

It was occasionally a close call, so did the Captain of the sub knew in advance the gap's size and that the sub has enough space?

Edit: After learning more about the Poles, the correct answer is that there is no continental shelf under the North Pole and the Arctic sea's depth is 4.2 km there. The ice is only about 2 meters thick so there is plenty of room. Due to global warming during the summer months, it is occasionally iceless now.

But getting there is another issue. The Nautilus had to give up its approach once because in the Bering Sea the ice's thickness was 18 meters and the sea floor was too high up due to a ridge, thus it didn't have enough room.

The second US submarine (USS Skate) reaching the North Pole breached the surface by breaking through the ice.
gpt
They had been using sonar both active and passive on submarines for a long time. They can hear how far it is from the bottom and the ice sheet. It was also definitely not the first venture of a sub under ice. Just further and longer than they have gone before.