But for Rhea Favonia, we might have lived with it. "There's a smelll There's a horrible smell. I'm not going in the rel

I didn't need to be an informer to know we were stuck. When a four-year-old girl reckons she has detected something nasty, you just give in and look for it. My little niece would not go near the bath house until we proved there was nothing horrible in the caldarium. The more we scoffed and told her the hot room was only smelly because of its new plaster, the more Rhea screamed hysterically at bath time. There was nothing visible, and the rest of us tried to ignore it. But the child's insistence unsettled everyone.

There was a faint odour. If I tried sniffing it out, I lost it. When I decided there had been nothing, straight away I smelled it again.

At least Helena and I were able to go home to our own new house. My sister Maia and her children had to stay on there on the Janiculan Hill, in the home that was supposed to be their refuge from trouble, living with that other kind of trouble, Pa. My father, Geminus, and I were in the throes of a house-swap. While I tried to organise decorators to renovate his faded old lair on the bank of the Tiber, he took over the spread on which I had already worked for months, where all that remained for completion was the new bath house.

The Janiculan house had a highly desirable location if you worked on the north side of Rome. It suited Pa, with his auction house and antiques business in the Saepta Julia by the Pantheon. My own work required free access to all parts of the city. I was an informer, serving private clients whose cases could take me anywhere. However much I wanted to move out and across the river, I needed to live close to the action. Sadly, this sensible thought had only struck Helena and me after we had bought the new house.

By chance, father's long-term companion, Flora, then died. He turned into a maudlin romantic, who hated the mansion they had shared. I had always liked the riverside quarter below the Aventine. Sowe organised an exchange. The bath-house contractors became Father's problem. That was appropriate because Pa had introduced them to Helena in the first place. I enjoyed waiting to see how he would persuade Gloccus and Cotta to finish, a task where even Helena had failed despite the fact she had been paying their bills. As with all builders, the more unreliable they had become, the more extortionate those bills were.

With Pa, we couldn't win: by some means, he fixed them. Within a week, Gloccus and Cotta had grouted their last wobbly tile and cleared off. My father then possessed a fine domestic outbuilding with a full cold room, tepid room, three-piece sweating-room suite; natty dipping pool; integral changing area with modish pegs and clothes bunkers; separate furnace and log store; de luxe Greek marble basins and a custom-designed sea-god medallion in one newly laid mosaic floor. But while people were admiring his Neptune, they also noticed the odd smell.

In moments when it caught me, that reek seemed to carry hints of decay. Pa knew it too. "It's as if the room had been locked up with some old codger dead inside for months."

"Well, the room's brand new and the old cove is still alive, unfortunately." I gathered Pa must have had some neglected neighbours, in the past life we never discussed. I myself knew about smells like that from other situations. Bad ones.

There came an evening, after a long hot day, when we found we could no longer ignore the stink. That afternoon I had been helping Pa dig over a terrace, Jupiter knows why. He could afford gardeners and I was not one to play the dutiful son. Afterwards, we both sluiced off. It must have been the first time we bathed together since he ran away when I was seven. Next time we met, I was home from the army. For a few years I even pretended not to know who he was. Now I had to tolerate occasional brushes with the old rogue, for social reasons. He was older; he was on his own with that, but I was older too. I now had two baby daughters. I should allow them a chance to learn to despise their grandfather.

As we stood in the hot room that evening, we faced decision time. During the day, I had done most of the heavy work. I was exhausted, yet I still rejected Pa's offer to scrape a strigil down my back. I made a rough job of cleaning off the oil myself. Pa favoured a concoction of what seemed to be crushed iris roots. Incongruous. And on that hot sultry night, nowhere near strong enough to mask the other smell. "Rhea right." I glanced down at the floor. "Something's rotting in your hypocaust."

"No, no; trust me!" Pa used the voice he kept for assuring idiots that some piece of Campanian fakery could be 'school of Lysippus', if looked at in the right light. "I told Gloccus to omit the hypocaust from this room. His quotation was outrageous for under floor work. I worked out some figures myself, and with that kind of area to heat, I was going to be spending four times as much on fuel…" He tapered off.

I eased my foot against the wide instep strap of a bath shoe. Helena's original scheme had involved properly heating the whole warm suite. Once she admitted what she was up to here, I had seen the plans. "What have you done then?"

"Just wall flues."

"You'll regret it, you cheapskate. You're on high ground. You'll find it chilly round your rude bits in December."

"Give over. I work right by the Baths of Agrippa." Entrance was free. Pa would love that. "I won't need to use this place except in high summer."

I stretched slowly, trying to ease the stiffness in my lower back. "Is the floor solid? Or had they already dug out a hypocaust when you decided against it?"

"Well, the lads had made a start. I told them to floor over the cavity and block off any links to the other rooms."

"Brilliant, Pa. So there won't be an access point for crawling under this floor."

"No. The only way in is down."

Nice work. We would have to break up the mosaic we had only just taken over brand new.

The under floor space in a usable hypocaust would be eighteen inches high, or two feet at most, with a mass of tile piers to support the suspended floor. It would be dark and hot. Normally they send boys in to clean them, not that I would inflict it on a child today- to face who knew what? I was relieved there was no formal access hatch. That saved me having to crawl in.

"So what do you think about this smell, Marcus?" my father asked, far too deferentially.

"The same as you. Your Neptune is; floating on rot. And it's not going away."

Instinctively we breathed. We caught a definite hum.

"Oh Titan's turds."

"That's what it smells like, Pa!"

We ordered the furnace slave to stop stoking. We told him to go to the house and keep everyone else indoors. I fetched pickaxes and crowbars, then Pa and I set about ruining the sea-god mosaic.

It had cost a fortune but Gloccus and Cotta had produced their usual shoddy work. The suspended foundation for the tesserae was far too shallow. Neptune, with his wild seaweed hair and boggle-eyed attendant squids, would soon have been buckling underfoot.

By tapping with a chisel, I identified a hollow area and we set to. My father got the worst of it. Always impetuous, he put his pick in too fast, hit something, and was spattered with foul yellowish liquid. He let out a yell of disgust. I leapt back and stopped breathing. A warm updraft brought disgusting odours; we fled towards the door. Judging by its powerful airflow, the under floor system must never have been blocked ofF completely as Pa ordered. We were now in no doubt what must be down there.