Winter has officially come for a select group of submarine sailors.

The Seawolf-class fast attack submarine Connecticut and the Los Angeles-class fast attack sub Hartford have arrived in the Arctic Circle for Ice Exercise 2018, or ICEX, a five-week training operation in the Arctic Ocean designed to assess the Navy’s readiness in frigid climates, a Navy release said.

The two boats will be joined by the Royal Navy’s Trafalgar-class submarine HMS Trenchant. Together, the subs will train in the icy waters, surface at the North Pole and collect scientific data.

The crews will also be tested with setting up an outpost called Ice Camp Skate, a temporary camp built on a sheet of ice that will house a command center, shelters and other infrastructure to support more than 50 sailors.

“With every ICEX we are able to build upon our existing experience and continue to learn the best way to operate in this unique and harsh environment,” said Rear Adm. James Pitts, the commander of the Undersea Warfare Development Center.

“We are constantly testing new tactics, techniques and procedures under the ice, and this exercise allows us to do so on a larger scale and alongside our U.K., joint and academic partners.”

U.S. subs most recently completed an ICEX in 2016.