At 2 a.m., oceanographer Ryan Smith was headed into his 12th hour of work with little sleep when trouble started.

From the rear deck of the University of Miami’s research boat, he guided the vessel’s winch to lower a cage containing 14 long, gray tubes, collectively weighing about 1,000 pounds, hundreds of meters deep into the Atlantic Ocean, to record the temperature, salinity and density of the water. But after running smoothly for the first two-thirds of the trip, the sensors now suddenly stopped transmitting data.

There was no time for a hiccup. With urgency mounting, Smith signaled to bring the cage to the surface.

At sea, there is no helpline to call for a broken instrument at this hour (or any hour). If the team couldn’t fix it, they would need to make a 12-hour slog back to Miami through the fast-moving Florida Current — the precise subject they were trying to measure.

For 43 years, scientists have been studying the strength of the water flow between Florida and the Bahamas to learn what drives its changes over time. The information could help scientists answer a pressing question: Is the Florida Current, one of the world’s fastest ocean currents, slowing down? If so, it could indicate weakening of the larger circulation system in the Atlantic Ocean — what scientists call the Atlantic Meridional Overturning Circulation (AMOC) — which could be disastrous.

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