The Doctor often takes us on adventures that double as history lessons, so when he landed the Tardis on a frozen river Thames at the end of series 10 episode 3 we knew things were going to get educational...
But why was the Thames totally frozen over? And what was that elephant doing plodding around on the river?
Sometimes truth can be stranger than fiction: it's time to learn all about the real-life Thames Frost Fairs.
What were the Thames Frost Fairs?
Between the mid-14th century and the 19th century weather temperatures in Britain were significantly lower. As a result, the period was nicknamed the Little Ice Age.
During this time the river Thames froze over on several occasions, and the ice was sometimes thick and solid enough to support a whole fair or festival. These events were known as the Frost Fairs.
The earliest recorded fair was held during the winter of 1607/8. In December 1607 the ice was thick enough to walk across, with many using the route to get from Southwark to the City. Cold temperatures persisted until January 1608, when the first stalls were set up on the ice by fruit-sellers, shoemakers, barbers and even publicans.
Shopkeepers sold goods and games were also played, with football and bowling on the ice.
Several more fairs were held in 1683/4 (when the Thames froze for two months), as well as 1716, 1739/40, 1789 and 1814.
Why did the river Thames freeze over? And why doesn’t it freeze over now?
Well, the cooler climate was partially to blame – as was the medieval London Bridge.
The old bridge across the river had numerous narrow piers, which would trap pieces of ice and dam up the river. As the water became shallow it froze over more easily.
The old London Bridge was demolished in 1831, allowing the water to flow more freely, and the river was 'embanked' in several stages throughout the 19th century. This, coupled with rising temperatures, made it difficult for the river to freeze over again.
Was there really an elephant on the Thames in 1814?
The 1814 Frost Fair was the last to be held on the Thames, though organisers couldn't have known that at the time. On 2nd February The Times reported that "in some parts the ice was several feet thick, while in others it was dangerous to venture upon".
That didn't stop traders from setting up their stalls and pubs, though. And to add a little something extra, an elephant was actually paraded across the ice near Blackfriars bridge.
The Doctor may have many fictional adventures, but this one in particular is most definitely based on fact.
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