Buried Genius: Why Some Ancient Cities Drained Better Than Ours Do

Every civilization that has ever packed a lot of people into one place has run into the identical, unglamorous problem: what to do with the water once it has been used, and where to put it so it does not sit in the street, breed disease, and make the place unliveable. It is one of the least glamorous questions in engineering and one of the most important, and the honest record shows that some of the oldest civilized answered it more completely than a great many rich, modern ones still manage.

A city built with the drain first

Start with the oldest and, on the evidence, the best of the bunch. Around 2500 BC, in the Indus Valley, a city archaeologists now call Mohenjo-daro was laid out on a grid, streets running at right angles, wide enough for carts, and here is the detail that still makes engineers pause: the drainage was not added after the city was built. It was part of the plan from the first brick. Every house, not just the grand ones, had its own bathing area, and its wastewater ran through a small drain in the wall out into a covered channel running beneath the street outside, lined with fitted, waterproof brick and sized with real precision, roughly two feet deep and three feet wide on the main runs. Those street channels fed into larger drains, which fed into even larger ones, all built to a falling gradient so that gravity, not machinery, did the work of moving the waste away. Manholes and sumps, brick-lined pits designed to be opened and cleared, sat at intervals so the system could be maintained rather than simply abandoned when it clogged.

This was not a single showpiece building. It was the ordinary infrastructure of an ordinary city, repeated house after house, for a population that numbered in the tens of thousands. Nothing built on that scale, for that many ordinary residents rather than a handful of elites, would exist again for a very long time.

The first flush

A different piece of the puzzle turns up a little over a mile of history later and a very different part of the ancient world entirely. Around 1700 BC, in the palace of Knossos on the island of Crete, someone built a toilet that could be flushed. An attendant or the user poured water through a hole in the floor outside the toilet room, which ran down a channel into a clay pipe beneath the seat, carrying waste away on demand rather than waiting for rain, which meant the thing worked even in the middle of a dry Mediterranean summer. Similar fittings have since been found in other Minoan palaces on Crete, connected to a wider network of stone and terracotta pipes that handled clean water, wastewater, and rain separately across the whole site. It is, on the current evidence, the earliest flush toilet anyone has found.

The drain that outlived the empire

The example most people have actually heard of, if they have heard of any of this, is Roman. Around 600 BC, under one of Rome's early kings, workers began lining a marshy stream bed with stone to drain the ground that would become the Roman Forum. Over the following centuries that channel was covered, extended, and rebuilt in stone as a proper enclosed tunnel: the Cloaca Maxima, Rome's "greatest sewer." By the height of the empire it was carrying off rainwater, street runoff, and the outflow of the city's public baths and latrines, all moving by gravity down to the Tiber.

Here is the detail worth sitting with. The Cloaca Maxima is not a ruin behind glass. Part of it still works. To this day it carries rainwater and debris away from the old Forum district, more than 2,600 years after the first stones went in. Almost nothing else built in that century is still doing its original job.

Why they got it right

Line up Mohenjo-daro, Knossos, and Rome and the shared trick is not some lost ancient technology nobody can rediscover. It is sequencing. Every one of these systems was built into a settlement's plan from the start, or close to it, rather than squeezed in afterward around buildings and streets that already existed. Drainage got the same gradient, the same materials, and the same priority as the walls and the roads, because the people laying it out understood, correctly, that a city that cannot get rid of its own water eventually cannot function as a city at all.

A modern city that tried, and still fell behind

Compare that with London, a genuine case of a city that took the problem seriously and still could not stay ahead of it. Through the summer of 1858, hot weather baked the untreated waste in the Thames into a stench so overpowering, remembered as the Great Stink, that it drove Parliament, meeting right beside the river, to finally fund a real solution. The engineer Joseph Bazalgette designed and built a network of 82 miles of enclosed brick main sewers and more than a thousand miles of connecting street sewers, completed in 1875, intercepting waste that had been running straight into the river and carrying it well downstream instead. It was, for its era, a genuinely serious piece of planning, and much of it is still the backbone of London's sewer network today.

It was also, eventually, not enough. Bazalgette built his system to cope with a fixed amount of rainfall and a Victorian-sized population. London outgrew both. The network mixes rainwater and sewage in the same pipes, so a hard storm can push more volume through the system than it was ever sized for, and when that happens the overflow is designed to spill straight into the Thames rather than flood the streets, which is an improvement on flooding but is still, functionally, an old city periodically discharging raw sewage into its own river. The fix, a new 25-kilometre storage tunnel called the Thames Tideway Tunnel, cost roughly five billion pounds and was not finished until 2025, a hundred and fifty years after Bazalgette's original network, to solve a version of the exact problem Mohenjo-daro's builders had already solved, permanently, in 2500 BC.

What four thousand years actually teaches

Set next to that, most modern urban growth looks even worse, because most modern cities never got a Bazalgette moment at all. They simply grew, often far faster than any single authority ever planned for, and drainage was left to arrive later, bolted onto streets and buildings that had already been built without it in mind, patched pipe by pipe rather than laid out gradient by gradient from a blank plan. That is precisely the sequence Mohenjo-daro's planners avoided five thousand years ago, and precisely the sequence that keeps producing modern streets that turn into rivers the moment the rain is heavier than usual.

None of this means the ancient world was cleverer than the modern one in some general sense. It built far less, moved far less far, and knew far less about the world than any city today. But on this one specific, unglamorous question, where does the water go, the oldest planned cities on record simply got the sequence right in a way a great many newer, richer, more technically capable cities still have not managed. Plan the drain first, and it can last four thousand years. Add it later, and you may still be paying for the fix in the 2020s.

Sources

• Wikipedia, "Sanitation of the Indus Valley Civilisation" (Mohenjo-daro's covered street drains, house-level bathing areas, and the roughly 2500 BC construction date).
• Toilet Guru, "Minoan Toilets, Drains, and Water Pipes" (the Knossos flush toilet, its mechanism, and its roughly 1700 BC date).
• Wikipedia, "Cloaca Maxima" (the sewer's origin as a 6th-century BC open channel, its later enclosure, and its continued partial use today).
• London Museum, "How Bazalgette built London's first super-sewer" (the 1858 Great Stink and Bazalgette's 1875 network).
• Wikipedia, "London sewer system" (combined sewer overflow, the Thames Tideway Tunnel's £5 billion cost, and its February 2025 completion).