Beach cleaners in Denmark were left astonished when they found a 'large strange creature' during a litter pick-up. 

The intriguing object, measuring 5 feet (1.5 metres), was washed up at Dueodde, a beach on Danish island Bornholm's southernmost tip. 

After picking it up, they noticed just how heavy it felt. 

However, after taking it to experts, the discoverers found the object was not any kind of 'sea snake' as initially believed.  

So can you guess what it actually is? 

It's a humpback whale's penis!

Beach cleaners René Vilsholm, Morten Gebhardt and Kenneth Nielsen had been picking up rubbish at their local beach on Monday, September 16. 

When they found the lengthy member, they contacted local conservation group NaturBornholm, which confirmed its true nature. 

'It was a different day at work,' NaturBornholm said in an Instagram post with photos of the penis. 

'A big weird creature washed up on land close to Snogebaek on Monday. 

'It turned out, after some speculation, to be a penis belonging to the young humpback whale that earlier this year.'

Experts think the phallus once belonged to a whale that washed up at the location earlier in the year – although it's unclear how exactly it detached. 

NaturBornholm, based in Aakirkeby, Denmark, plans further analysis on the fleshy package, but for now it is being kept in a freezer. 

It told MailOnline that it has already cut a slice of the penis in an effort to get 'a better insight into the organ'. 

A humpback whale’s penis can be up to 10 feet long and is normally concealed inside the animal's genital slit – located close to the whale's belly. 

Tom Mustill, wildlife filmmaker and whale enthusiast, told MailOnline: 'There’s nothing I know about that would make a penis more likely to drop off.

'Like whales testicles and mammary glands, they are inside the whales – keeping them insulated and streamlined.

'My bet would be the whale died or was killed and the penis was extruded after death (pushed out by pressure from decomposition building up inside) and then fell off in decomposition.'

Witnessing a live whale's penis extrude from its genital slit is rare, according to Pacific Whale Foundation, a nonprofit organisation based in Maui, Hawaii. 

The foundation already released footage of a humpback whale that was swimming with its penis extruded – which it describes 'a rare and fascinating sight'. 

As the video shows, the male humpback whale is swimming towards another whale with his penis clearly visible. 

But it is not known if the humpback whale being approached is male or female.

Pacific Whale Foundation claims males can direct their penis towards either males as well as females – possibly as some kind of threatening territorial display.

Adult humpback whales (officially known as Megaptera novaeangliae) range in length from 39 to 52 feet and weigh about 36 metric tons. 

The males produce a complex song lasting 10 to 20 minutes, which they repeat for hours at a time, although both males and females use some form of vocalizations. 

Along with the blue whale, the fin whale and others, the humpback whales is a baleen or 'filer feeder'.

Baleen whales have a filter-feeding system inside their mouths instead of teeth, which they use to collect shrimp-like krill, plankton and small fish from the sea. 

In comparison, toothed whales, such as the beaked whale and sperm whale, bite their food or even swallow it whole.