Dozens of shipwrecks, the first of their kind seen from bygone empires, have been a "complete bonus" discovery for researchers mapping the Black Sea.

The international team of scientists came across more than 40 wrecks while surveying the seabed near Bulgaria to understand how quickly land in the area was inundated following the last ice age 20,000 years ago.

"The wrecks are a complete bonus, but a fascinating discovery, found during the course of our extensive geophysical surveys," Professor Jon Adams, lead investigator on the Black Sea Maritime Archaeology Project, said.

Some wrecks found, including this one, were from the Ottoman Empire. ( Supplied: Rodrigo Pacheco-Ruiz )

The centuries-old wrecks, including ships from the Byzantine and Ottoman empires, provide the first view of vessels previously known only through historical sources.

Most other wrecks from these time periods have decomposed due to having sunk in shallower waters.

But the Black Sea discoveries can be seen with their upper decks still intact, with some ships' masts still standing and rigging in place.

Professor Adams said these wrecks were "astonishingly preserved" due to the lack of oxygen in the Black Sea below 150 metres.

The stern of this Ottoman-era wreck was "astonishingly preserved" due to the lack of oxygen in the Black Sea below 150 metres. ( Supplied: Rodrigo Pacheco Ruiz and Rodrigo Ortiz )

The wrecks were found using advanced underwater survey systems aboard the Stril Explorer.

Images of the underwater discoveries were created by combining thousands of photos to create an accurate 3D model, using millions of points to align each photo in a process called photogrammetry.

"We've been able to capture some astonishing images without disturbing the sea bed," Professor Adams said.

"Certainly no-one has achieved models of this completeness on shipwrecks at these depths."

Professor Adams said the ships were likely trading ships rather than battle ships, due to their positions far out at sea.