The USS Nautilus was part of the the US Navy Arctic Submarine Laboratory, which undertook numerous submariner explorations of the Arctic.
The research culminated in the transpolar submerged voyage of USS Nautilus in 1958 and included scores of under-ice cruises to gain scientific knowledge essential to Arctic submarine operations.
The Soviet Union did a similar exercise to the USS Nautilus expedition in 1962.
Following the 1958 voyage of USS Nautilus, the Leninskiy Komsomol (named for Vladimir Lenin's Komsomol) traveled under the Arctic ice and surfaced at the North Pole on 17 July 1962.
US submarine activity is still on going.
Two U.S. Navy fast-attack submarines will surface near the North Pole over the course of the exercise (see video below of the first of these, the USS Toledo, surfacing on March 4, 2020). The submarines will also conduct several transits through the region.
The US is not the only country to have submarines in the Arctic, it's also part of Russia's backyard.
Non-military organizations are also exploring the Arctic,
Since 1997, the International Bathymetric Chart of the Arctic Ocean (IBCAO) has been the authoritative source of bathymetry for the Arctic Ocean.
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Many satellites fly over the arctic that can measure it's drift speed and ice depth. 
Here are some of the journeys of sonar craft measuring the seafloor
