The slime that was found at the Ham Wall Nature Reserve in Somerset earlier this year (PA)
"The slime is still a genuine mystery," said Chesca Rogers, who has been leading the attempts to identify the gelatinous material.
"There are stories in folklore that link it with meteor sightings. Some people think it might be unfertilised frog spawn, others think it is a fungus, or a slime mould or that it is plant related.
"None of the tests we have done so far have told us anything conclusive, but the samples we got were not in the best condition and highly contaminated."
Some of the items brought into the Identification and Advisory Service, which is based inside the museum's Angela Marmont Centre for UK Biodiversity, are quickly ruled out as not being all they may first seem.
Members of the public have brought in lumps of stone they believe could be the fossilised remains of a dinosaur head, potential meteorites that turn out to be waste slag from iron works or balls of tinfoil and insects that could be new species.
A skull of a Chinese Water Deer was mistaken for that of an ice-age sabre tooth cat (NHM)
Three years ago they received a skull still with fur and flesh attached - that the person who found it thought could be the remains of a recently dead sabre tooth cat.
Rather than being the modern day relative of these ferocious ice age predators, however, it was in fact the remains of a more docile Chinese Water Deer.
From photographs of the skull, it is easy to understand the mistake these deer have long tusks that protrude from their upper jaw that look like they could be incisors.
Then there have been dragon skulls. These turn out to be pelvises of sea birds such as auks or puffins.
The pelvis bones of sea birds like auks and puffins are often mistaken for dragon skulls due to their strange shape (NHM)
John Tweddle, manager of the Angela Marmont Centre for UK Biodiversity, said: "We receive a lot of objects that look like something else.
"We get flint objects that can form some really interesting shapes that people think are fossilised footprints or skulls, but they are just an unusual shape.
"Sea bird pelvises can look like the right shape for a dragon's skull, so we get people saying they think they have found a dragon."
On some occassions they receive items of dubious origin - such as an "alien" embamned in a jar of fluid. It turned out to be a science fiction toy that had been kept on a shelf of a pub.
Toy alien that was believed to be a monkey. (RII PHOTOGRAPHY)
"The person who brought it to us thought it might be a monkey of some sort," said Mr Tweddle. "We are still not sure if it was a joke, but it is the closest we have had to an alien being brought in.
"Generally even if these things are not as interesting as they first seem, these are still all interesting objects and we would never discourage someone from coming forward.
"Here in the UK we get a few new species being discovered every year, so there is every chance of finding something new in this country."
The centre was recently involved in identifying the first population of false wolf spiders, Zoropsis spinimana, in the UK
They also receive large numbers of insects such as beetles, hornets and bees, all carefully packaged in an array of match boxes, chocolate boxes, tubes and playing card packets.
Few turn out to be anything other than a large example of a British species queen hornets are commonly mistaken for the more aggressive Asian hornet that has spread in mainland Europe.
However, it is the Somerset slime that would probably have been of most interest to Fox Mulder and Dana Scully in the X-Files.
It appeared at the RSPB's Ham Wall Nature reserve during the same week as a meteor exploded in the skies above Chelyabinsk, in Russia.
Speculation that the jelly like substance had an extra-terrestrial origin heightened after an amateur photographer announced he had captured on film a mysterious object in the sky above the reserve itself in the same week.
The strange streak looked like another meteor but was not confirmed by astronomers.
Tony Whitehead, a spokesman for RSPB Ham Wall, said: "We had all sorts of theories about what this could be.
"If you look back in the records there are references to 'star jelly' and astromyxin that date from the 16th century so this phenomenon has been linked to space for a long time.
"There are many more rational options that have been put forward, such as the jelly used by frogs to coat their eggs that is expelled when they are speared by birds, but we still don't know for sure."
Now a new batch of slime has been discovered at the same site this week, at a time when frogs are not spawning, deepening the mystery further. Fresh samples are now being sent to the museum's experts for analysis.
Dr Hanna Hartikainen, a researcher at the museum who has been conducting experiments on the original slime samples, said: "We had a look under the microscope but it doesn't really have a much of a structure it is a gelatinous matrix.
"We extracted some DNA from the jelly and tried to identify where it came from using probes for frog and bird DNA but were not able to find any of those.
"We also used a new technique called Next Generation Sequencing and found a lot of DNA from nematode worms, fungi and bacteria but none that could be the source of the slime, so they were probably just colonising it.
"It is still an unsolved mystery."
But like all true X-Files stories the truth is out there. You just need to know where to look.
An Afterhours event at the Natural History Museum about the Mystery of the Intergalactic Slime and the other work conducted at the Angela Marmont Centre will be held on 25 October at 7pm.