Over time, it is not unusual for ancient structures to become buried and neglected. However, in the bustling streets of Lisbon, a hatch opens twice a year to reveal a 2,000-year-old Roman system still holding up the buildings above it.

Dating back to the A.D. first century, the "crypto portico" underground maze of tunnels and passageways was built by the Romans, who occupied the city known as Olissipo beginning around 200 B.C. The city remained under Roman control for several centuries.

"This structure guaranteed and, 2,000 years later, continues to guarantee that the buildings above our heads are stable and safe for those who live, work and walk up there," said Joana Sousa Monteiro, director of the Lisbon Museum, as she toured the site known as the Roman Galleries.

It opens for only a few days in April and September each year. The space is usually flooded due to an aquifer running beneath the city. The water, which is essential for its preservation, must be pumped out to allow access.

The galleries were first discovered in 1771 when Lisbon was rebuilt after the devastating Great Earthquake of 1755.

Tickets to visit the galleries usually sell out within 15 minutes. Among the lucky ones who managed to buy one was Gustavo Horta, a Brazilian who lives in Lisbon.

"It's unmissable," he said shortly after climbing the steep stairs from the underground galleries. "I've waited two years to go on this tour."