(CNN) -- The search for Malaysia Airlines Flight 370 is now in its 40th day, and searchers have yet to turn up a piece of the plane. Here's the latest to catch you up on the search efforts:
What's the latest with the search?
The Bluefin-21 probe being used to search for the missing jetliner in the deep waters of the Indian Ocean returned to duty Tuesday after its first mission ended prematurely. The probe surfaced early in its first deployment after encountering waters that exceeded its current 4,500-meter (14,764-foot) maximum depth. What was supposed to be a 20-hour mission ended in less than eight, according to a source, and found nothing. An official with the U.S. Navy told CNN's "The Lead with Jake Tapper" that engineers believe the Bluefin-21 probe can go as deep as 5,000 meters (16,404 feet) with some software adjustments.
Does that mean something's wrong with the probe?
Not at all. In fact, the probe did exactly what it's programmed to do when the ocean floor dips below its maximum depth, said David Gallo, director of special projects at the Woods Hole Oceanographic Institution. "The vehicle's tracking the floor, so when the floor dives, so does the vehicle. And the vehicle goes, 'Uh oh, I'm not supposed to be here' and punches up," he said.
But what if the plane is in deeper waters?
"Well, if the Bluefin cannot bring back the kind of sonographic images they want and the information that they want, then they're going to have to move on to the next level of vehicles with names like Alvin, Remora and the Sea Dragon, and those can go deeper," said CNN aviation consultant Mary Schiavo. "The Sea Dragon can go down to 4.4 miles, or 7,000 meters. And that's the next step that you have to do, is go down to the level in a different kind of vehicle."
Why aren't they listening for pings from the plane's 'black boxes' anymore?
Because the batteries powering the boxes' locator beacons are probably dead, according to the manufacturer and other experts. The batteries were supposed to last at least 30 days, and the plane has now been missing for 39 days. The batteries could have continued powering the beacons for a few more days but almost certainly have run out by now. Searchers using devices to listen for the pings went six days without hearing anything, so they are now focused on the underwater search.
What happens if they find the black boxes?
The flight data recorders, or FDRs, would be transferred to fresh water and then dried before the data they contain would be extracted, Schiavo said. "Then they'll discover on the FDR what they're dealing with and how much of the wreckage they really have to bring up to solve the mystery."
But don't hold your breath. The Flight 370 search is often compared to the hunt for wreckage from Air France Flight 447, which plunged into the South Atlantic Ocean in 2009, killing all 228 people aboard. It took investigators nearly two years to recover the black boxes in that case.
Is the surface search continuing?
It is, but maybe not for much longer. Eleven military aircraft, three private planes and 11 ships participated in the surface search Wednesday about 2,087 kilometers (1,297 miles) northwest of Perth, Australia, according to the country's Joint Agency Coordination Centre. The center's director, retired Air Marshal Angus Houston, said Monday that the surface search is likely to end in the next few days.
Has that search turned up anything?
Searchers found an oil slick in the area over the weekend and are shipping a 2-liter sample back to Australia for analysis. If it's oil typically used in aircraft, the slick could be an important lead. But it may not be. A slick found in the early days of the search for Flight 370 turned out to be fuel oil from a freighter.
Any other new details?
A U.S. official told CNN on Monday that co-pilot Fariq Abdul Hamid's cell phone was on and made contact with a cell tower in Malaysia about the time the plane disappeared from radar.
However, the U.S. official -- who cited information shared by Malaysian investigators -- said there was no evidence Fariq had tried to make a call.
The details do appear to reaffirm suggestions, based on radar and satellite data, that the plane was off course and was probably flying low enough to obtain a signal from a cell tower, the U.S. official said.