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CHILD flushes a pet baby alligator down the toilet. Elsewhere in the city, a chemicals company pumps effluent containing growth hormone into the sewer. The result: a 10-metre alligator bursts through the pavement, unleashing murder and mayhem. It is the classic script for a B movie, which made its way onto the world鈥檚 screens in 1980 as Alligator.

Hollywood has mined a rich vein from our image of the sewers as a stinking subterranean labyrinth, and our fears about what might lurk within. In the 1988 remake of The Blob, for instance, cosmic radiation mutates microorganisms on board a satellite into a mass of pink jelly which craves human flesh. When the spacecraft crashes back on Earth, the creature slopes off to the sewers. From this under world base, it emerges from the town鈥檚 drains and to terrorise local residents.

Like many of Hollywood鈥檚 flights of fancy, however, some of its unlikely sewer tales contain a grain of truth. In 1984, a live baby Nile crocodile was pulled out of the Parisian sewers. Today, it lives in more desirable surroundings at the Tropical and Oceanographic Aquarium in Vannes, Brittany. And in Florida, alligators quite regularly crawl into the storm drains. Dennis David is section leader of the Florida Game and Fresh Water Fish Commission鈥檚 alligator management programme. He receives some 10 000 鈥渘uisance alligator鈥� complaints each year. Most incidents happen above ground, but around a dozen involve alligators in the culverts beneath roadside drains. These animals are quickly removed, to avoid tragic child-alligator confrontations. 鈥淜ids play around the drains and they begin poking sticks and throwing rocks at them,鈥� says David.

Snakes, meanwhile, may turn up almost anywhere. Two years ago, the pipe carrying solids away from the screens that protect the plant鈥檚 pumping equipment at Northumbrian Water鈥檚 Howdon sewage works jammed. When Dave Erikson, a worker at the plant, opened the screen chamber to diagnose the problem, 3 metres of reticulated python fell out onto his head. A colleague, Eddie McLoughlin, helped him wrestle the creature to the floor, although neither knows whether the snake was still alive. 鈥淚 thought it moved, so I hit it with a shovel,鈥� says McLoughlin.

The Northumbrian python incident is just one of many. Snakes like exploring confined spaces and think nothing of a short underwater swim, so a toilet鈥檚 U-bend is no obstacle 鈥� as several anguished snake keepers have found to their cost. Few of these animals are seen again. But last August, a man in the Swedish city of Lund lifted the lid of his toilet to find a boa constrictor curled up inside the bowl. The snake had disappeared down a neighbour鈥檚 toilet, and made its way through the drains.

Other animals end up in the sewers because of human cruelty. Goldfish can regularly be seen swimming along the sewers of Brighton, says Andy Gates of Southern Water, who runs the resort鈥檚 sewer system. And while the Ninja turtles of film and television fame are pure fantasy, terrapins do turn up in the sewers. Red eared terrapins soon grow from coin-sized children鈥檚 pets into vicious carnivores that can give a nasty nip. At that point, parents often simply flush the problem away. 鈥淯sually, they鈥檙e swimming along to their heart鈥檚 content,鈥� says Gates, although most will soon be swept into the treatment works and meet an unpleasant end.

Strange incidents like these provide sewer workers with a rich source of anecdotes, but they reveal little about the sewer as a functioning ecosystem. So what of the effluent-loving creatures which make our drains and sewers their permanent home? Unsurprisingly, few biologists chose to make the sewers their life鈥檚 work, and consequently we have a better idea of what roamed around some Roman sewers than many of today鈥檚. Archaeologists have catalogued a rich insect fauna which lived in a sewer under York鈥檚 city centre during Roman times. The insects included the aquatic larvae of psychodid flies, which also inhabit the filter beds of modern sewage works. Tiny spines called spicules that come from sponges also appeared in Roman sewage. But these animals were not sewer dwellers: a sponge on a stick was the Roman equivalent of toilet paper.

If there is one animal inextricably linked in our minds with the sewers, it is the brown rat, Rattus norvegicus. The sewer rat鈥檚 horrendous public image, however, is not entirely deserved. Sewer rats have, in general, received little scientific attention. But in the 1950s, they came under the scrutiny of a team of researchers from the Ministry of Agriculture, Fisheries and Food who roamed the sewers of several English towns and cities, laying baits of grain as they went. Surprisingly, their surveys showed that there was no clear relationship between the density of rat populations in the sewers and the incidence of infestations on the surface immediately above. This belied the common perception that rats spotted above ground have usually emerged from the sewers.

Since then, researchers have studied sewer rats simply by laying baits beneath manholes. This gives an incomplete picture, as the I950s work showed that some sewer rats never visit manholes. But the newer research has at least confirmed one thing: sewer rats and surface rats are not synonymous. While surface populations are if anything increasing, the sewer rat is on the wane 鈥� in London at least. Last year, Dave Cowan of MAFF鈥檚 Central Science Laboratory in Slough was commissioned by Thames Water to put grain baits and piles of sawdust down 5 per cent of London鈥檚 250 000 manholes. Where the grain was taken, but the sawdust was left behind, indicating that the bait had not simply washed away, he recorded an infestation. This suggested that 13 per cent of manhole shafts are visited by rats, whereas a similar survey in the late 1970s recorded a figure of 23 per cent. The reasons for this decline are unclear, although Cowan suspects it may be due to improved sewer maintenance blocking many of the holes used as nesting sites. Whatever the reason, the results indicate that the danger posed by sewer rats has been overestimated. 鈥淲e have to get the risk in perspective,鈥� says Cowan. 鈥淎bove-ground rat infestations are much more likely to pose an immediate health hazard.鈥�

The one section of the population who is potentially at risk from pathogens carried by sewer rats are the 鈥渇lushers鈥� who spend their working days trudging our sewers, checking their condition and clearing any blockages. Rat urine contains a bacterium called Leptospirosa icterohaemorrhagiae which can enter the human body through any mucous membrane or wound. At first, it causes flu-like symptoms. But in some cases, this mild illness develops into a life-threatening infection called Weil鈥檚 disease, which causes jaundice and kidney failure. Historically, this condition was called sewer worker鈥檚 disease. But today, protective clothing has all but stamped out this occupational hazard. Between 1990 and 1992, the 39 recorded cases of L. icterohaemorrhagiae infection in England and Wales included only one sewer worker. Today, the people at greatest risk of contracting Weil鈥檚 disease are water sports enthusiasts. But the rats responsible for these cases are almost certainly those living on the banks of lakes and rivers, rather than sewer dwellers, as L. icterohaemorrhagiae should not survive sewage treatment.

If the sewer rat does not deserve its bad press, another famous sewer scavenger certainly does. Cockroaches overrun many of the world鈥檚 sewer systems, particularly in warmer climates. Sewer-dwelling cockroaches are teeming with organisms that cause human disease. 鈥淭hey literally feed on anything that鈥檚 floating past,鈥� says Nicholas Burgess, a medical entomologist at the Royal Army Medical College in London. And that includes human faeces: Burgess has shown that cockroaches can survive perfectly happily on a diet consisting entirely of human faeces and used medical swabs. The problems come if the insects launch nocturnal forays up through the drains and into areas where food is prepared. Like houseflies, they vomit over their food to soften it before tucking in. Cockroaches also defecate as they dine.

Thankfully, most British sewers are free of cockroaches. But the insects will thrive wherever a large amount of hot water enters a sewer, keeping it warm and humid. The sewers beneath restaurant districts, large hotels and hospitals are favourite haunts. Burgess surveyed London鈥檚 sewers for cockroaches in the 1970s. Appropriately enough, he found that Soho鈥檚 Chinatown hosted a population of Blatta orientalis, the oriental cockroach, which likes temperatures above 20 掳C. The sewers beneath one of London鈥檚 plushest hotels, meanwhile, were frequented by a different class of cockroach altogether. With their laundries and enormous kitchens, luxury hotels can keep the sewers immediately beneath at a tropical 30 掳C and 95 per cent relative humidity. As American tourists enjoy the facilities above, the American cockroach, Periplaneta americana, flourishes in the warmth below.

While cockroaches merely pose a threat to human health, some sewer dwellers undermine the fabric of the sewer itself. Sewage is a rich broth of microorganisms. 鈥淵ou can almost say, 鈥榣et鈥檚 list everything鈥�, and you鈥檙e likely to get it in a sewer,鈥� says Eddie Bellinger, a microbiologist at the University of Manchester. Most of these organisms are little studied. The exceptions, however, are those that produce noxious gases. Several types of bacteria, including Methanothrix and Methanosarcina, produce methane and can thrive in the oxygen-starved sludge that builds up in slow-flowing sections of sewers. These bacteria obtain their energy by breaking down volatile fatty acids such as acetic acid to form methane and carbon dioxide but fortunately there is little risk that methane could build up to the levels needed to trigger an explosion.

An unusual microbial double act, however, poses a real structural threat to many sewers. First, bacteria in the living 鈥渂iofilm鈥� of microorganisms which lines the walls of a sewer below water level convert sulphates in the sewage into hydrogen sulphide. Again, this chemical reaction gives the bacteria energy and happens only in parts of the biofilm that are starved of oxygen. Bacteria like Desulfoto maculum nigrificans are sulphate specialists.

Hydrogen sulphide is a serious problem in itself. In small quantities, the gas advertises its presence through the classic 鈥渞otten egg鈥� smell of stagnant sewage. At higher concentrations, however, this odour disappears, and the gas is as toxic as hydrogen cyanide. For this reason, sewer flushers continually monitor the atmosphere as they travel through the system.

The threat to the structure of a sewer, however, arises when a group of bacteria called thiobacilli get to work on the hydrogen sulphide. These bacteria live on the roof, or crown, of the sewer, above the water level. They add oxygen to the hydrogen sulphide, converting it to sulphuric acid. In Victorian brick-built sewers, this acid can corrode the mortar between the bricks; in modern concrete pipes, the acid can eat away the entire sewer crown.

Because bacteria perform their chemical tricks more rapidly when warm, this corrosion worsens as the temperature rises. British sewers are generally too cool for the bacteria to do much damage, but in Southern California, it is a different story. The Sanitation Districts of Los Angeles County, which manage some 1900 kilometres of sewer, about 1000 kilometres of them made from concrete pipes, spend up to $20 million each year monitoring, preventing and repairing the damage caused by sulphuric acid corrosion.

Left to its own devices, the 10 centimetres of concrete lining the roof of a typical Los Angeles sewer pipe can corrode all the way through within 20 years. Roads overhead may well collapse, sending cars crashing into the sewer. To tackle the problem, the LA County authorities have developed an automated float which sprays the sewer crown with magnesium hydroxide. In effect, says David Greenwood, the sanitation districts鈥� sewer supervising engineer, the pipes are dosed with 鈥渋ndustrial strength milk of magnesia鈥�.

Even when sewer-dwelling microorganisms finally get swept out of the system and into the treatment works, they can still wreak havoc. The behaviour of one group of sewer bacteria, in fact, makes them worthy of a walk-on part in The Blob. In recent years, the settlement tanks at many sewage works have been overcome by a brown foam that is so stiff cats can walk across its surface. 鈥淚t is known in the trade as chocolate mousse,鈥� says Nigel Horan, a public health engineer at the University of Leeds. The foams are sometimes so prolific that they climb out of the tanks. 鈥淚t can go walkabout,鈥� says Christopher Forster of the University of Birmingham, who has studied the problem.

Bacteria which multiply to form long filaments cause the foams. Fast growing colonies trap bubbles of air and form the mousse-like films. The culprits are thought to be soil bacteria which are washed into the sewers. In Britain, the main villain is Microthrix parvicella; while in the US, most foams have been attributed to bacteria called Nocardia 鈥� although some microbiologists believe that the American foams are due to two closely-related genera called Gordona and Tsukamurella.

The biology of all of these foam-forming bacteria is largely unknown, so researchers have little idea why they have only recently become such a nuisance. One intriguing theory pinpoints an increase in our consumption of polyunsaturated fats. The first chocolate mousse was reported in Milwaukee nearly 30 years ago 鈥� about the time that health-conscious Americans began to substitute vegetable oils for lard and other animal fats. This change, some scientists speculate, has made human faeces a more appealing source of nutrients for the foam-forming bacteria. So when you next sit down and consider whether to spread butter or margarine on your morning toast, spare a thought for the bacteria that will dine on your sewage.