The Rome Index
Aula Ottagona (Octagonal Hall)
A perfectly preserved Roman dome hidden inside a modern Italian building near Termini.
What it is
Part of the Baths of Diocletian complex, the Aula Ottagona is an octagonal domed hall from the early 4th century that later became a church, then a planetarium, and now forms part of the Museo Nazionale Romano — housing Roman bronze sculptures in a space almost entirely unchanged since antiquity.
Why visit
Free entry (or included in the Museo Nazionale Romano ticket), completely uncrowded, and one of the most legible examples of Roman interior architectural scale in the city. The acoustic qualities of the dome are also striking.
Location
Five minutes walk from Termini station, making it an ideal first or last stop if you're arriving or departing by train.
Its unlikely 20th-century use
Between its life as part of the Baths of Diocletian and its current museum function, the Aula Ottagona spent decades in the 20th century operating as a planetarium — a striking, largely forgotten chapter that saw an ancient Roman domed hall repurposed for modern astronomical projection before its eventual conversion to display Roman sculpture.
The acoustic quality worth testing
The octagonal dome creates a genuinely distinctive acoustic effect — speaking or even just walking creates an audible resonance that's rare to experience in a space this historic and this quiet, worth pausing for a moment of stillness to actually notice rather than moving straight through.
