People think one piece of Amy Bradley evidence is key to solving entire case

A Netflix documentary delving into the disappearance of Amy Bradley - who vanished on a cruise ship nearly three decades ago - has left viewers baffled and some are asking the same thing over one key detail.

The 23-year-old, from Virginia, in the US, had been enjoying a Royal Caribbean cruise holiday with her parents, Iva and Ron, and her brother, Brad, when she vanished from her room.

The documentary - which dropped on Netflix earlier this month - examines the countless theories, witness testimonies, footage and police investigations that followed Amy's disappearance.

But unfortunately, after almost three decades, she has not been found.

While there are a number of conspiracies about where Amy is now - and why she has not been found - many viewers have been left with one resounding question, which they claim could answer the mystery.

Amy went missing in 1998 (Netflix)

And it's all to do with her shoes - a pair of 'Birkenstock-like' sandals that were discovered on the balcony of her cabin.

What happened to Amy Bradley?

Amy had been travelling on the Rhapsody of the Seas cruise ship when she went missing.

The ship was on its way to Curacao on 24 March when Amy was last seen by her family.

That night, she had been out for a formal dinner before she decided to head out with her brother for a pool party and limbo contest.

Her brother Brad explained the pair had been at the ship's disco when he decided to go back to their room.

"I went out onto the balcony, about five minutes later Amy came back to the room. We were both finishing our drinks, hanging out and talking about the next day," he said.

"She brought up that someone she had been dancing and talking to during the course of the evening made some sort of physical pass at her. She told me it was the bass player from the band."

While Brad went to bed, Amy explained she wanted to stay on the balcony as she wasn't 'feeling too good'.

Brad went off to bed, but at 5.30am, Amy's dad Ron woke up and saw Amy's legs on a lounger on the balcony.

But when he woke again around half an hour later, at approximately 6.00am, he discovered she was no longer there.

"Something woke me up again, I don't know what it was but when my eyes opened, I looked out again and she wasn't there," said Ron.

"The balcony door was open about 12-14 inches, I noticed her cigarettes weren't out there and the shirt she was wearing that night was laying on the chair in her room.

"The first thing I'm thinking is she's changed her clothes and went up, getting a cup of coffee, taking pictures because we were coming in to port."

This was the beginning of the Bradley family's nightmare.

What's the theory behind Amy Bradley's shoes?

In the documentary, viewers learn that the shoes Amy had been wearing - a pair of sandals similar to Birkenstocks - remained on the balcony after she went missing.

While her cigarettes were gone, many pointed out that if she had left the room, it would make more sense for her shoes to be missing, too.

But, could she have worn a different pair?

Many following the case believe details around Amy's shoes could be the key to cracking her disappearance, with one person writing on Reddit: "For me, what happened to Amy really boils down to that. If there were no shoes missing, it would indicate to me that she never left the cabin and likely went overboard. If there are shoes missing, along with her cigarettes and camera, I could be persuaded that she left the room between 5:30-6:00AM and somehow met with foul play."

While another added: "That’s the single most important bit of information. No woman is leaving the cabin without her shoes on. She would have just put her sandals back on."

And a third said: "The answer is very simple. If all of her shoes were accounted for, she fell off. If a pair of shoes is missing than she willingly left."

"I want to know if any of her other shoes are missing?" asked another.

Amy Bradley and her mother Iva (Netflix)

How many pairs of shoes did Amy Bradley take on the cruise and were any missing?

It's not clear how many pairs of shoes Amy took on the cruise - although some reports online suggest she took up to 10.

However, her brother has given a definitive answer about what the family do know over on Twitter, answering the question: "Brad, can you confirm if another pair of Amy’s shoes were missing? Or were the Birkenstocks on the balcony her primary ones?"

He responded: "She brought several pairs of shoes, but we don’t know which ones or how many she brought, so we can’t determine which are missing."

Amy went missing from a Royal Caribbean cruise (Netflix)

What have Amy Bradley's family said about the shoes?

While Brad has explained they aren't sure how many shoes she took on the cruise, it's reported that her family have previously said she packed at least 10 pairs.

However, it remains they have no idea which ones Amy took and therefore aren't sure if any are missing.

While many have said that Amy would likely grab the sandals on the balcony if she was about to head out, others have said it's plausible that she could have grabbed a different pair if she left the room.