The 270-tonne bell, believed to be one of the largest ever cast, was made on the order of King Dhammazedi in 1476 and donated to the revered Shwedagon pagoda. In the early 1600s, it was stolen by Portuguese despot Philip de Brito. The vessel carrying the historic treasure sank where the Yangon and Bago rivers meet the Pazundaung creek.
Private and foreign groups have tried unsuccessfully to retrieve the bell in the past. Buried deep beneath the mud, they have been deterred in part by murky waters and torrential currents.
More than 70 divers - including 10 "sea gypsies" from the country's Myeik archipelago, famous for their ability to dive deep without external breathing equipment - were taking part in the latest mission, said Win Myint, the 52-year-old expedition organizer.
They have made exploratory dives over the last several days, but because of heavy silt and mud on the riverbed have not been able to locate the bell, and were set to dive again on Thursday, Mr Myint said.
He said he has always dreamt of salvaging the Dhammazedi Bell and returning it to the Shwedagon pagoda.
As divers wearing goggles and attached to safety ropes jumped into the water, Buddhist monks in a separate boat prayed for their safety, Mr Myint said, expressing confidence that the mission would end in success.
Mr Myint added the project would last up to 45 days and would cost about $200,000, most of which had come through donations.