How deep is 2.8 miles? It's deeper than an inverted Statue of Liberty (305 feet), deeper than an inverted Eiffel Tower (1,063 feet), deeper even than an inverted Burj Khalifa, the tallest building in the world (2,717 feet).
(CNN) -- Investigators got new information that may help them narrow the possible whereabouts of Malaysia Airlines Flight 370 as details emerged Thursday that shed light on the doomed flight's final moments early March 8 as it flew from Kuala Lumpur toward Beijing.
A search plane has detected a possible signal from the locator beacons on the so-called black boxes from the missing aircraft, the Australian agency coordinating the search announced.
"The acoustic data will require further analysis overnight but shows potential of being from a man-made source," said retired Air Chief Marshal Angus Houston, the agency's chief coordinator.
Other important elements were revealed Thursday from a senior Malaysian government official and another source involved in the investigation:
• Malaysia Airlines Flight 370 disappeared from military radar for about 120 nautical miles after it crossed back over the Malaysian Peninsula, sources say. Based on available data, this means the plane must have dipped in altitude to between 4,000 and 5,000 feet, a senior Malaysian government official and a source involved in the investigation tell CNN.
Former U.S. Department of Transportation Inspector General Mary Schiavo cited two possible reasons for the plane's reported dip: to get out of the way of commercial traffic, which typically flies at higher altitudes, and to reach a level where pressurization is not needed and those aboard the plane would have been able to breathe without oxygen in the event of a loss of pressure.
The dip could have been programmed into the computers controlling the plane as an emergency maneuver, said aviation expert David Soucie.
• Malaysian air force search aircraft were scrambled around 8 a.m., soon after Malaysia Airlines reported that its plane was missing early March 8, Malaysian sources told CNN. The aircraft were scrambled before authorities corroborated data indicating that the plane turned back westward, a senior Malaysian government official told CNN.
• But the air force did not inform the Department of Civil Aviation or search and rescue operations until three days later, March 11, a source involved in the investigation told CNN.
• MH370's pilot, Capt. Zaharie Ahmad Shah, was the last person to speak to air traffic controllers with the words "Good night Malaysian three-seven-zero," sources told CNN. The Malaysian sources said there was nothing unusual about the voice and there was no indication of stress. One of the sources, an official involved in the investigation, told CNN that police played the recording to five other Malaysia Airlines pilots who knew the pilot and co-pilot. "There were no third-party voices," the source said.
The possible signal heard by a search plane was picked up through sonar buoys equipped to receive such electronic data and was detected near the Australian ship Ocean Shield, said the Joint Agency Coordination Centre.
Crews have been narrowing the search area in the Indian Ocean.
Up to 10 military aircraft, four civil aircraft and 13 ships were to assist in Thursday's search for the Boeing 777-200ER, which was carrying 239 people when it vanished March 8 on a fight from Kuala Lumpur, Malaysia, to Beijing.
Three of the vessels -- the Ocean Shield to the north, and the British HMS Echo and Chinese Haixun 01 to the south -- were focusing under water.
Aircraft and ships spotted a number of objects during Wednesday's search, but could recover only a small number, none of which appeared linked to MH370, the JACC said.
Thursday's search area is about 22,400 square miles (58,000 square kilometers), centered some 1,417 miles (2,280 kilometers) northwest of Perth. That's roughly the size of West Virginia.
But the latest search area is about three quarters of the size of the area that teams combed the day before and far smaller than what it was a few weeks ago.
Thursday's underwater search area was bracketed by the Ocean Shield at the northern end and the Chinese ship Haixun 01 and HMS Echo at the southern end.
The Ocean Shield first picked up two sets of underwater pulses on Saturday that were of a frequency close to that used by the locator beacons. It heard nothing more until Tuesday, when it reacquired the signals twice. The four signals were within 17 miles of one another.
"I believe we are searching in the right area, but we need to visually identify wreckage before we can confirm with certainty that this is the final resting place of MH370," Houston said Wednesday.
In another piece of encouraging news, authorities analyzed the signals picked up over the weekend and concluded that they probably came from specific electronic equipment rather than from marine life, which can make similar sounds.
"They believe the signals to be consistent with the specification and description of a flight data recorder," Houston said. "I'm now optimistic that we will find the aircraft or what's left of the aircraft in the not too distant future."
Thursday is Day 34 in the search. Time is of the essence: The batteries powering the flight recorders' locator beacons are certified to emit high-pitched signals for only 30 days after they get wet.
"The signals are getting weaker," Houston said Wednesday, "which means we're either moving away from the search area or the pinger batteries are dying."
CNN's Nic Robertson and journalists Ivy Sam and Chan Kok Leong in Kuala Lumpur contributed to this report.
posted by Greg Palmer