The unearthed tunnel-aqueduct system, which was cut out of the bedrock 15 metres beneath the ground, stands about two metres above the ground and is 1,305 metres long, according to Mostafa Waziry, the secretary-general of the Supreme Council of Antiquities.
Two alabaster heads of a sphinx and an unknown statue from the Ptolemaic period were also found.
The Taposiris Magna temple was built by Pharaoh Ptolemy II Philadelphus between 280 and 270 BC.
The Graeco-Roman Period (332 BC -395 AD) marked the end of Persian rule over Egypt.
The mission announced the discovery, which was made during excavation two months ago, after the completion of all studies.
Martinez told Ahram Online that the tunnel-aqueduct underground Taposiris Magna is an exact replica of the Eupalinos Tunnel in Greece, which is 1,036 metres long and runs through Mount Kastro in Samos Island, Greece.
The Eupalinos Tunnel is considered one of the most important engineering achievements of antiquity and a UNESCO Cultural Heritage Site in Greece.
It was built in the sixth century BC to serve as an aqueduct and supply fresh water and refuge to the city of Samos.
“Taposiris Magna Tunnel is an engineering feat of outstanding importance," Martinez said.
"It is 270 metres longer than the Eupalinos Tunnel and runs from Lake Mariout to the Mediterranean Sea," Martinez said.
"Inscriptions were found on a wall inside the tunnel of Taposiris Magna and they are currently being studied,” she pointed out.
Part of the tunnel was found on the Mediterranean Sea bed by the underwater archaeological team led by the Dominican University of Santo Domingo in cooperation with the Egyptian underwater archaeological team of Alexandria.
Archaeological evidence has shown part of the Taposiris Magna complex collapsing and sinking in the Mediterranean Sea.
For the first time, a submerged portion of the complex was discovered through visual survey.
The first underwater archaeological survey led by Martinez brought outstanding results.
This survey unearthed sunken structures under heavy sediments of mud, including the first limestone rectangular blocks, pottery and amphoras.
In the previous archaeological seasons, the mission made important discoveries that may shed light on the Graeco-Roman Period in Alexandria.
The expedition found many artefacts in the temple such as coins with the portrait and names of Queen Cleopatra and Alexander the Great.
It also uncovered a foundation deposit that proves the main sanctuary temple of Taposiris Magna was built by Ptolemy IV and dedicated to goddess Isis.
Several headless statues were also uncovered in the temple as well as inscriptions and various busts and statues of the goddess Isis.