I remember a rat catcher I interviewed for a local paper I was working on. He told me about how sometimes you'd get rats int the sewers and they'd crawl up into the houses round the U bend. But me made me promise not to put it in the story because it would upset people.

So be thankful if you've got alligators down there to keep the rats in check.

Incidentally the alligator legend has a precedant - here is an extract from Henry Mayhew'sLondon Labour and the London Poor:

There is a strange tale in existence among the shore-workers, of a race of wild hogs inhabiting the sewers in the neighbourhood of Hampstead.

The story runs, that a sow in young, by some accident got down the sewer through an opening, and wandering away from the spot, littered and reared her offspring in the drain, feeding on the offal and garbage washed into it continually. Here, it is alleged, the breed multiplied exceedingly, and have become almost as ferocious as they are numerous.

This story, apocryphal as it seems, has nevertheless its believers, and it is ingeniously argued, that the reason why none of the subterranean animals have been able to make their way to the light of day is, that they could only do so by reaching the mouth of the sewer at the river-side, while, in order to arrive at that point, they must necessarily encounter the Fleet ditch, which runs toward the river with great rapidity, and as it is the obstinate nature of the pig to swim against the stream, the wild hogs of the sewers invariably work their way back to their original quarters, and are thus never to be seen.

What seems strange in the matter is, that the inhabitants of Hampstead never have been known to see any of these animals pass beneath the gratings, nor to have been disturbed by their gruntings. The reader of course can believe as much of the story as he pleases, and it is right to inform him that the sewer-hunters themselves have never yet encountered any of the fabulous monsters of the Hampstead sewers.

I was going to type this in - but I checked, via Google, and found someone had already done it for me - together with this note, and the quotes below:

The quote above is from Volume II of the 1861 edition of Mayhew's London Labour and the London Poor (London: Griffin, Bohn, and Company). The writings that make up London Labour were originally published between 1849 and 1851 in the Morning Chronicle newspaper. The sewerswine were also briefly mentioned in a Daily Telegraph editorial of October 10, 1859.

Mayhew also wrote of rumors of killer rats in the sewers:

Other stories are told of sewer-hunters beset by myriads of enormous rats, and slaying thousands of them in their struggle for life, till at length the swarms of the savage things overpowered them, and in a few days afterwards their skeletons were discovered picked to the very bones.

And later:

Moreover, far from there being any romance in the tales told of the rats, these vermin are really numerous and formidable in the sewers, and have been known, I am assured, to attack men when alone, and even sometimes when accompanied by others, with such fury that the people have escaped from them with difficulty. They are particularly ferocious and dangerous, if they be driven into some corner whence they cannot escape, when they will immediately fly at any one that opposes their progress. I received a similar account to this from one of the London flushermen.

And, quoting one of his informants:

"I've often seed as many as a hundred rats at once, and they're woppers in the sewers, I can tell you; them there water rats, too, is far more ferociouser than any other rats, and they'd think nothink of tackling a man, if they found they couldn't get away no how, but if they can why they runs by and gits out o' the road.

"I knows a chap as the rats tackled in the sewers; they bit him hawfully: you must ha' heard on it; it was him as the watermen went in arter when they heard him a-shouting as they was a rowin' by. Only for the watermen the rats would ha' done for him, safe enough. Do you recollect hearing on the man as was found in the sewers about twelve year ago?--oh you must--the rats eat every bit of him, and left nothink but his bones. I knowed him well, he was a riglar shore-worker.

"The rats is wery dangerous, that's sartain, but we always goes three or four on us together, and the varmint's too wide awake to tackle us then, for they know they'd git off second best."