You must have seen it. At the south-eastern corner of London Bridge, with Duke Street Hill beneath it, 16m-high light-grey Portland Stone pointy thing. A lot of people know it as 'the spike'.
However, there is no plaque or sign telling you what the spike is, or what it may represent. In fact, it may well be London’s most mysterious pointy thing.
And as always happens in the absence of any explanation, people have come up with their own.
One idea is that the spike is a giant sundial, yet it points 19.5 degrees to the south west — too shallow to make it work.
Many others think back to the gory history of the site and old London Bridge (1209-1831), the medieval crossing that had gatehouses at either end. Displayed on those gatehouses, as warning to anyone who was thinking of bringing trouble into the City of London, were the heads of executed criminals skewered onto spikes.
The rebellious Scot William Wallace's head is thought to be the first to be pinned there. The leader of the Peasants' Revolt Jack Cade's head ended up there, as did those of Catholic martyrs Thomas More and Bishops John Fisher, and Protestant reformer (and Hilary Mantel's muse) Thomas Cromwell.
In 1598 a German visitor to London called Paul Hentzner counted counted over 30 heads on iron spikes at the south end of the bridge.
So is the spike an unspoken memorial to the victims of this brutal rite? The Past in the Present blog describes it as a fitting memorial to the beheaded victims.
Free Tours by Foot state confidently "the spike is a visual reminder and a commemoration of the brutal end to some notorious lives", while countless public speakers and tour guides point to the spike and tell the story of the severed heads.
It doesn't stop there. Beer aficionado Pete Brown, in his (otherwise excellent) book Shakespeare's Local writes "the spike commemorates a far more ancient local design feature. A grisly quirk gone wrong," before writing evocatively of 16th and 17th century London and the dead heads gazing down on Southwark.
Which is all very convincing and conclusive except for an article that appeared in the May 2014 issue of the Fortean Times by David Hambling. Pondering the meaning of the spike, he contacted Eric Parry Architects, who actually built the thing in 1999, and asked them of its significance.
Is it really a memorial to decapitation victims of yore?
Even a writer as wise as Pete Brown, among many others, has unknowingly participated in telling an urban myth. Architect José de Paiva told Hambling that there is no esoteric or literal symbolism to the work.
The spike — its actual name is the Southwark Gateway Needle by the way — is a marker. It is tilted at 19.5 degrees to point down to a specific spot. Follow the trajectory of the needle downwards and it points to the riverside opposite the City church St Magus the Martyr. This is where the medieval London Bridge crossed the river.
The bridge built afterwards (1831-1970) was built beside it. The Southwark Needle is not, as many have surmised, a memorial to the heads spiked on old London Bridge, but a marker for the bridge itself.
It is also a reminder that London’s landscape can still generate mystery and myths in the 21st century.