Archeologists are trying to solve the mystery of a massive Roman building which has been unexpectedly unearthed in Hungary's capital, Budapest.

The massive structure emerged during the construction of a new housing development, and now baffled archeologists are trying to decipher what purpose it could have served.

What these archeologists from Budapest's Aquincum Museum are asking is how did the ruins of a building this big manage to stay hidden for so long?

So far they can only speculate about the function of this vast Roman-era structure.

"If we look only at the floor plan, our thoughts can go in quite a few different directions," says Gergely Kostyál, the lead archeologist of the Aquincum Museum.

It's emerged on the site of one of Budapest's largest housing developments where the former Óbuda Distillery once stood.

The Roman-era building is believed to be of great significance and is approximately 40% intact despite people having lived in this area for nearly two millennia.

The excavation has revealed a long, narrow building, presumed to be several hundred meters long.

It ends in a tower-like, polygonal structure.

According to archeologists at the museum, the discovery could be exciting for the international scientific community as well.

During the first phase of the excavation, from March to the end of May this year, the sections that were uncovered were reburied to protect the walls.

An aerial photograph clearly shows that two long, straight walls, the corridor, were attached to the 'tower' on one side.

The narrow building can be seen running parallel to the Danube with the polygonal tower-like structure to the north. Its southern extent is still unknown.

Some sections of the wall are as tall as a person – a remarkable sight in Hungary, where most Roman ruins reach no higher than knee height.

"It must have been a public building that we found," says Kostyál. "I'm sure that it wasn't a private house. You can tell this just from its size alone that it has some kind of public function. It's also certain that the soldiers stationed here have built it."

The ancient city of Aquincum lies on the northern part of today's Budapest. The 50,000-square-meter site of the former Óbuda Distillery extends over the north eastern quarter of the Aquincum military town and its northern outskirts.

A test excavation to assess the site's archeological significance was conducted in the summer of 2017. Preliminary findings revealed that the structure could have been built at the end of the 3rd or during the 4th century AD.

"Based on the quality of the building, we also can conclude that this was a military construction project. Which is not surprising, since in the late Roman period, only the army could undertake a construction project of this scale," says Tibor Budai Balogh, another archeologist at the Aquincum Museum.

The Romans reinforced the walls of both the corridor and the tower from the outside with pillars.

The handiwork of the Roman army's construction units can be recognized in the professional design of the structures. But Kostyál says he doesn't believe it's a military enforcement.

"We must definitely rule out the possibility that this was a fortress, since the walls are too thin for that. Walls of 65 cm wide are not sufficient for a fortification. They were too thin even back then,” he says.

According to the archeologists, early Roman fortifications often had walls 1.5 meters wide and they cite examples of late Roman legionary fortresses at the empire's frontiers from the 3rd and 4th centuries, which had walls three and a half meters wide.

Kostyál says "It's also interesting that this building doesn't appear on any maps or drawings from that period. We have checked all three available military maps, and it wasn't on any of them either.

"But since these walls were so high, in my opinion, they must have remained above ground, even sometime after the Roman era. So somehow, they must have been visible later on as well. But we do not have any written records of them."

The city has already decided the site will receive archeological protection so it can be preserved for the future.

Maps of ancient cities have long since fallen into ruin so each excavated element is like a piece of a giant jigsaw puzzle.

Research can only take place where the fabric of the Hungarian capital is incomplete or broken up by new developments – like the Waterfront City housing project on the site of the former Óbuda Distillery.

The research on this plot of land will help the archeologists to add another square to the map of Aquincum.