A young female journalist boarded an inventor's submarine. Then she vanished.

On Thursday afternoon, freelance journalist Kim Wall stepped aboard the UC3 Nautilus, an imposing, privately built, black submarine docked in the Port of Copenhagen.

The 30-year-old Swede was there to interview its owner, a Danish inventor named Peter Madsen – an eccentric man famous for building the sub via crowdfunding in 2008, and for his company RML Spacelab, which plans to be the first to launch a human being into space in an amateur-built rocket.

But by Friday afternoon, the 46-year-old man was being winched to safety by a nearby boat as the Nautilus sunk to the depths of Koge Bay. Technical issues, he claimed; a “minor problem with a ballast tank”, he told local station TV2.

But with Kim Wall nowhere to be found, authorities had their suspicions. Suspicion enough that, on Saturday, Madsen was charged with manslaughter.

So what happened that evening?

Kim Wall boarded the submarine at around 7pm on Thursday, reportedly to conduct an interview for Wired Magazine. There are photographs - the last known to have been taken of the Swede - that show her standing with Madsen on the vessel's tower that evening, looking out to sea.

According to a police spokesman, Madsen claimed to have dropped the journalist at Refshale Island at 11.30pm. However, her boyfriend reportedly alerted police on Friday morning that she hadn't returned.

A wide-scale air, land and sea search has so far failed to locate the missing woman.

What do police allege happened to her?

According to Associated Press, prosecutor Louise Pedersen told a packed courtroom on Saturday that Madsen was facing preliminary charges “for having killed in an unknown way and in an unknown place Kim Isabell Frerika Wall of Sweden sometime after Thursday 5 pm.”

Police suspect the UC3 Nautilus was deliberately sunk. However, when the vessel was pulled from the water on Saturday and there was no sign of the journalist's body.

Police have refused to reveal what, if any, evidence was found inside.

What happens now?

A judge on Saturday ordered that Madsen be held in pre-trial detention for 24 days while authorities continue their investigation.

If tried and found guilty, Madsen would face between five years and life in prison, according to Associated Press.

In the meantime, Wall's family and friends are desperately appealing for information that may help locate the prolific writer.

What it's like when a loved one goes missing. Loren O'Keeffe shares her story. (Post continues after podcast.)

"It is with great dismay that we received the news that Kim went missing during an assignment in Denmark," her family said in statement given to The Associated Press.

According to her LinkedIn profile, Wall has been based in China and the US, and has been published by several outlets including Harpers, The Guardian and The New York Times.

She reportedly studied at Sorbonne University in Paris, the London School of Economics and in 2013 achieved a Masters in Journalism from Columbia University in New York.