What is the latest on the rescue effort?
US and Canadian teams are racing against the clock to find and rescue the small sub that went missing on Sunday.
The US Coast Guard's Rear Admiral John Mauger said on Monday that it is a challenge to conduct a search in such a remote area.
The search has two aspects, either on the ocean's surface or an underwater sonar search, he said.
The Coast Guard has sent two C-130 Hercules aircraft to search for the submersible on the surface of the water, and has been joined by a Canadian C-130, and a P8 aircraft equipped with underwater sonar capability. Sonar buoys are also being deployed in the area.
Rear Adm Mauger said it would need additional expertise to rescue the vessel if it was found underwater and he has reached out for help.
Where is the Titanic wreck?
The wreck of the Titanic is located about 600km (370 miles) off the coast of Newfoundland, Canada, and it sits 3,800m (12,500ft) deep at the bottom of the Atlantic.
The passenger liner hit an iceberg on its maiden voyage from Southampton to New York in 1912. Of the 2,200 passengers and crew onboard, more than 1,500 died.
The remains of the Titanic were discovered in 1985, and the site of the wreck has been extensively explored since then.
It lies in two parts, with the bow and the stern separated by about 800m (2,600ft). A huge debris field surrounds the broken vessel.
