Everything you need to know about the search so far for the missing sub

Hot News.

A submersible carrying five people to see the wreckage of the Titanic at the bottom of the North Atlantic Ocean is still missing despite a massive search operation – but banging sounds were reportedly heard in the area, according to a US governmental memo.

If you're just catching up, here's a recap of the developments so far:

About the excursion: The eight-day journey conducted by OceanGate Expeditions, priced at $250,000 a person, is based out of Newfoundland, with participants first traveling 400 nautical miles to the wreck site, which is about 900 miles (1,450 kilometers) off the coast of Cape Cod, Massachusetts. On this particular mission, the submersible, known as Titan, was carrying a pilot and four passengers.

How the sub went missing: The vessel began its two-hour descent to the wreck on Sunday morning. It lost contact with the Polar Prince, the support ship that transported the vessel to the site, 1 hour and 45 minutes into its descent, officials said. Search operations began later that day. It’s still not clear what happened to the submersible, why it lost contact, and how close to the Titanic it was when it went missing.

A race against time: Less than a day of breathable air may be left on the vessel, based on agency officials’ latest estimate. Some parts of Titan are decidedly low-tech. Unlike a submarine, a submersible needs a mother ship to launch it, has fewer power reserves and can’t stay underwater as long. The ship communicates with the vessel by text messages, and it’s required to communicate every 15 minutes, according to OceanGate Expeditions’ archived website. “All those things we’re used to now – GPS, Wi-Fi, radio links – do not work under the ocean,” according to former Navy submarine officer Capt. J. Van Gurley.

Developments in the search efforts: As the craft’s oxygen dwindles, banging sounds have been picked up from the Atlantic Ocean, signaling “continued hope of survivors,” according to a US government memo. The banging first came every 30 minutes and was heard again four hours later, the internal government memo obtained by CNN states. It was unclear when exactly the banging was heard Tuesday or how long it lasted, based on the memo. Rolling Stone was first to report news of the noise.

The tools being used in the search: The underwater sounds were detected Tuesday by sonar devices deployed by a Canadian P-3 aircraft to find the 21-foot vessel that lost contact Sunday. The US is moving in military and commercial assets as aircraft from the Canadian Armed Forces, the US Coast Guard and the New York Air National Guard continue to look above and below water, and France’s president has dispatched a research ship with an underwater robot to join the search Wednesday.

There were safety concerns: OceanGate touted Titan's safety features, despite conflicting information over its development. CNN has learned of at least two former OceanGate employees who expressed safety concerns about the vessel’s hull years ago, including the thickness of the material used and testing procedures. The 23,000-pound craft is made of highly engineered carbon fiber and titanium. In fact, technology used in the missing Titan sub is so new that it hasn't been reviewed.