Search for missing flight MH370 continues 4 years later with submarine drones A Texas-based seabed intelligence firm uses submarine drones in latest search.

Exactly four years after the fateful Malaysia Airlines Flight 370 departed Kuala Lumpur for Beijing, what happened after the plane and its 239 people on board finally came to rest remains one of the world's great aviation mysteries.

The flight is presumed to have landed somewhere in the southern Indian Ocean. Investigations and search efforts have been focused on an area called the 7th arc, where the aircraft is considered to have exhausted its fuel and descended into the water west of Australia.

The mystery has left the aviation world desperate to know where the aircraft is, why it went missing and how it has evaded investigators for years.

A Texas-based seabed intelligence company called Ocean Infinity is currently using submarine drone technology, which is more advanced in efficiency and precision than what was used in previous searches, to try to answer some of those questions.

But, so far, it hasn't returned any results either, despite covering an area Australian scientists targeted "with unprecedented precision and certainty."

The company's search was conducted as part of a "no-find, no-fee," 90-day contract with the Malaysian government.

Ocean Infinity is using a 378-foot vessel named Seabed Constructor with 65 crew members from Ocean Infinity, two Malaysian Navy officials and 8 autonomous underwater vehicles armed with cameras and sensors.

Multiple drones search the seafloor in a grid and send data back to investigators for analysis. If anything appears to resemble aircraft debris, they take a closer look. So far, nothing of significance has been found.

This may sound like a familiar story.

An initial 120,000 square kilomenter search led by the Australians lasted two-and-a-half years and cost $150 million before officials decided they were looking in the wrong place. That search ended in January 2017.

At that time, Australian investigators said they were ending their seafloor search, but pointed to a still-unsearched 25,000 square kilometer area further north.

Ocean Infinity had expressed interest in the search -- and were willing to pay for the operation themselves, unless they found the airliner. The company said any recovery process would be left to the governments. In the event the company finds MH370, the could be paid as much as $70 million. They launched in January 2018.

On Tuesday, Seabed Constructor had covered 23,000 of the 25,000 square kilometers highlighted by the Australians, according to a company spokesperson.

Sources close to the search operation told ABC News the company plans to remain at sea, searching, until the 90 days are up near the end of April.

Multiple theories have surfaced to explain how the jet vanished with relative invisibility, but there have been no definitive answers.

Authorities analyzed the Captain's home flight simulator and revealed the pilot practiced a flight similar to that of flight MH370: A Boeing 777 flight from Kuala Lumpur that traveled along the Strait of Malacca before turning left into the Southern Indian Ocean. The simulated flight traveled until fuel exhaustion before ending in the Southern Indian Ocean, according to the Australian Transport Safety Bureau.

"The MH370 captain’s flight simulator showed someone had plotted a course to the southern Indian Ocean," the Joint Agency Coordination Centre said in a statement in July of 2016. "The simulator information shows only the possibility of planning. It does not reveal what happened on the night of the aircraft's disappearance, nor where the aircraft is located."

The search efforts since the day of the flight MH370's disappearance have stretched the boundaries of science, but so far have been fruitless.

It's unclear how much investigators could learn in the event Ocean Infinity finds the jet. Even if the black box is retrievable and functional after years on some the ocean's deepest floors, the cockpit voice recorder operates on a loop and the mysterious early moments of the flight likely would have been erased.

However, one key question may be answered if the cockpit is found: Who was in control during the plane's final moments?

As the world observes the anniversary Thursday, families and friends of those on board flight MH370 are attending events and coming together online to grieve the loss of the 239 souls. Four years later, they still do not know exactly what happened.