‘Lake Albano is not a natural lake,’ Bertie said. ‘It’s a volcanic crater that’s fed by rainfall. This means that, unlike a regular lake, it has no natural outlet for the accumulated rainwater to escape.’

They came to a Roman ruin at the shoreline.

It was a large and complex structure, a square-sided brick building that delved into the forested hillside of the shore.

At its core was a straight sunken stone channel that allowed water to flow from the lake into a tunnel in the rear wall of the ruin. A gloriously high stone arch towered over the whole scene.

The ruin was well preserved and jaw-droppingly beautiful.

‘The Romans were very skilled builders, especially when it came to moving water,’ Bertie said. ‘This lake, like several others in the region, occasionally overflowed the crater’s walls and flooded nearby towns. So the Romans constructed an outlet to control the level of the lake and release water when necessary. That’s what this ancient structure is. It’s called an emissary.’

Iolanthe shook her head. ‘Albano’s emissary isn’t a person, it’s a Roman ruin.’

‘Correct,’ Bertie said.

The stonework of the emissary was shot through with dozens of gnarled tree trunks and snaking roots. Through the leafy canopy above it all, they could see the fading orange sky.

It looked like an idyllic grotto from a fairytale.

‘It’s in amazing condition,’ Nobody observed. ‘Got to give it to the Romans. They built things to last.’

‘I’ve seen pictures of this place before,’ Iolanthe said. ‘Old pictures.’

Bertie nodded. ‘Luigi Rossini and Giambattista Piranesi did detailed etchings of these ruins. I prefer Rossini’s, as they give a clearer image of the incredible masonry, but Piranesi’s engravings are quite lovely, too.’

‘It all looks like a giant drain,’ Nobody said.

‘That’s exactly what it is,’ Bertie said. He indicated the tunnel in the inner wall of the grotto: the sunken channel disappeared into it. A person could fit inside it but it was a tight squeeze.

‘That tunnel runs for over a kilometre at a steady descending grade. It was built around 400 B.C.E. It took thousands of slaves six years to excavate it, digging through the crater wall. It was terribly dangerous work—’

‘Bertie,’ Iolanthe said gently. ‘Like Nobody, I love your history lectures, but we’re racing to save the world here. What are we going to find?’

‘Right, right.’ Bertie hurried into the ruin, stepping quickly over the roots and vines, heading for the tunnel.

‘The Pope said he sent the Church’s notes about the location of the Blue Bell to Albano’s emissary. Now, Castel Gandolfo is one of the Church’s most secure places. This emissary lies deep beneath it.