Just a few miles from here is a district of London we'd all like to live in if we only had the cash. It's called Hampstead - up on a leafy hill to the north of the city. If you're an actor, a writer or an artist - and rich - it's the ideal place to settle down.
Among Hampstead's attractions are the little lakes, tucked away amid the trees on Hampstead Heath. There's the Ladies pond - where Katherine Hepburn used to go swimming in the 1960's - and the Mens' pond, which Gary Oldman used in a scene from one of his recent movies. You don't have to be famous to take a dip, but these days you do have to be brave. Not only are the Hampstead ponds cold and muddy - they have also become the adopted home to a fast breeding species of American critter capable of biting the unsuspecting butts of anyone who ventures in.
These things are three and a half inches long - they are hungry and aggressive. The Red Swamp Crayfish, also known as the Louisiana crayfish, has mysteriously arrived in Hampstead and is rapidly colonising the poshest ponds in the capital. The female lays six hundred eggs in one session. They've driven out our native species and already eaten most of the bigger fish too. Hampstead has an angling club. One member recently caught 180 crayfish in an hour. Creepy.
And all to the strange background screech of parakeets - because Hampstead Health also has a colony of these creatures living wild. They escaped from a London film set seventy years ago and survived in the undergrowth. But Parakeets come from the Himalayas and don't do any harm - unlike your wretched Red Swamp Crayfish that have now taken to nibbling bathers.
I should also mention that Hampstead allows bathing entirely naked. It is apparently very popular to strip off and dive in wearing nothing at all! However crayfish go for the ... er ... extremities -- which may explain those hideous cries echoing across Hampstead Heath -- and a big boost in the sale of strong wetsuits.
This is Ed Boyle for CBS News in London