The search for the missing submersible on an expedition to view the wreckage of the Titanic passed the critical 96-hour mark Thursday when breathable air could have run out, a grim moment in the intense effort to save the five people aboard.

The Titan submersible was estimated to have about a four-day supply of breathable air when it launched Sunday morning in the North Atlantic — but experts have emphasized that was an imprecise approximation to begin with and could be extended if passengers have taken measures to conserve breathable air. And it’s not known if they survived since the sub’s disappearance.

Rescuers have rushed ships, planes and other equipment to the site of the disappearance. On Thursday, the U.S. Coast Guard said an undersea robot sent by a Canadian ship had reached the sea floor, while a French research institute said a deep-diving robot with cameras, lights and arms also joined the operation.

Authorities are hoping underwater sounds might help narrow their search, whose coverage area has been expanded to thousands of miles — twice the size of Connecticut and in waters 2 1/2 miles (4 kilometers) deep.

The Titan was reported overdue Sunday afternoon about 435 miles (700 kilometers) south of St. John’s, Newfoundland, as it was on its way to where the iconic ocean liner sank more than a century ago. OceanGate Expeditions, which is leading the trip, has been chronicling the Titanic’s decay and the underwater ecosystem around it via yearly voyages since 2021.

By Thursday morning, hope was running out that anyone on board the vessel would be found alive.

Many obstacles still remain: from pinpointing the vessel’s location, to reaching it with rescue equipment, to bringing it to the surface — assuming it’s still intact. And all that has to happen before the passengers’ oxygen supply runs out.