Most people visit London to enjoy the view from the top of the London Eye, shop in Harrod's magnificent halls or tramp the passageways of the British Museum. There is so much to see. But tourists tend to overlook what is beneath their feet.

Sometimes with good reason.

It's been said that no one in London is ever more than three feet from a rat. There are tens of millions of rats in the great city, far outnumbering the people who blissfully ignore them. As recently as Victorian times, residents thought nothing of emptying chamber pots out the windows to join the bodies of dead animals, rotting vegetation, garbage and fish market offal. Cattle, sheep, pigs and horses deposited 40,000 tons of dung on London streets every year. In a poem about the River Fleet, Ben Johnson wrote: "The sinks ran grease, and hair of measled hogs."

In the 1840s, covered sewers frequently had no access to the Thames. There was such a stench from the sewer running beneath Fleet Street that it had to be reconstructed and eventually the Fleet River was completely covered over. The very best addresses in London, Westminster, Belgravia, Grosvenor, Hanover and Berkley Squares all smelled like the rankest offal, due to stopped up house drains. Buckingham Palace was one of the worst offenders, with sewers sometimes hundreds of years old and crumbling. Sewers were primitive, restricted to surface water and were quickly overwhelmed by 250,000 overflowing cesspits. When a new tunnel system was being built in the 1860s, all crafted by horse-and steam-driven cranes, the old sewers had first to be traced and charted because no one knew where they ran, which certainly gives an idea of how frequently they were cleaned. If there are so many rats today, it's nearly inconceivable to imagine how many of the animals basked in the abundance thrown their way in the Victorian Age.

But along with those rats, there is also a tremendous amount of fascinating history beneath the feet of London's tourists. There is evidence of bathhouses, amphitheaters, temples, taverns and markets. The layers of clay, gravel and stone have delivered up a Roman ship sunk by a stone cannonball and even a mammoth. Peter Ackroyd writes in Thames : The Biography of ancient bridges as well. A bridge over the Thames at Eton has been dated to ca. 1400 BC. Sometimes later structures were built on the foundations of earlier ones. A wooden bridge, also of Bronze Age provenance has been found near Vauxhall.

Mysteries abound beneath these cobbled streets. Bones are everywhere in London, constantly being exposed by builders and infrastructure repairs, from old cemeteries, prison gallows, hospital plots and pauper burial grounds, sometimes dating back to medieval and even Roman times. The Pauper's Cemetery at St. Bride's Church had been in use since Charles II's day. Bones were piled in heaps. Once a week, the remains of paupers were thrown into a hole fourteen feet deep. A clergyman said a few words and the grave received a slim covering of loose soil. The next week, the ground was opened up and a new lot interred. The whole neighborhood reeked with the smell of death.

A dozen rivers flow beneath the city, most completely covered over. One of the lost rivers of underground London is the Walbrook, which played a key role in the Roman settlement of Londinium. The town was established by the Romans after the invasion of 43 A.D. led by the Roman Emperor Claudius. The Walbrook got its name because it flowed right through the old walled city and into the Thames. The Romans even built a temple to Mithras on the east bank. It was discovered and excavated during reconstruction after World War II. Today the river flows completely underground, paralleling a street called Walbrook, not far from the Bank of England.

Londinium began as a small fort at the north end of a new bridge across the Thames. It became an important trade center between the Roman provinces of the continent and Britain. Just a few years after it was established, Londinium was sacked by a neighboring tribe called the Iceni led by Queen Boudica. Archaeologists have uncovered evidence of widespread destruction by fire at this time, in the form of a red layer of ash beneath the city. Also uncovered have been numerous examples of goods imported from across the Roman Empire.

The Tyburn River runs underground from South Hampstead through St. James's Park to the Thames at Pimlico near Vauxhall Bridge. The Tyburn gallows was alongside the river. It was the principal place of execution in the county of Middlesex from the 12th to 18th centuries, before they moved it to Newgate Prison. Bones and skulls sometimes washed out of the grounds.

Some underground sites still resonate with tourists today. Winston Churchill's underground cabinet war rooms are a popular visit. Tens of thousands sheltered here during the war in an alternate city not altogether dissimilar from those today beneath the cities of Toronto and Montreal. One gets a sense of what life was like here as the bombs of the Blitz rained down. Narrow corridors and primitive conditions were endured with representative British stoicism, and one can almost imagine Churchill's voice ringing out as he called to one of his secretaries, his cigar smoke wafting through the cramped hallways.

Another popular tour is a pair of pumping stations, magnificent edifices, complete with minarets. The Prince of Wales actually visited the opening of Crossness Pumping Station. The band of the Royal Marines played as his royal highness and his entourage of archbishops, princes, dukes and earls examined the boiler house and were taken into the culvert that connected to the sewers. It was constructed of superb brickwork and lined with rows of colored lights. One can just imagine that august assemblage strolling where there would soon be sewage up to their eyeballs.

Tube stations, war rooms, hidden rivers, Roman ruins, ancient bones and uncountable numbers of rats are just the tip of the iceberg when it comes to the secrets of undergound London.