It sounds like a line out of a horror movie. On Friday Ryanair flight FR7312 rapidly lost altitude and had to make an emergency landing in Germany, and over 30 passengers were taken to hospital.

Some of the passengers were bleeding out of their ears, while others had headaches, ear aches and nausea.

During a flight from Dublin to Croatia, the cabin became depressurised, with the air becoming cold, shortly followed by the deployment of oxygen masks you always hear about in those safety briefings before each plane trip.

According to the Australian Broadcasting Corporation, a website called shows the flight descending from 37,000 feet to 10,000 feet (11 to 3 km) in a terrifying seven minutes.

So why would this cause bleeding ears?

Cabin air pressure, when it's done correctly, sits at around 6,000-8,000 feet (1.8 to 2.4 km) above sea level.

Although this sounds high, it's much below the normal 36,000-40,000 feet (11 to 12.2 km) of typical cruising altitude, so there's a discrepancy between the air inside the cabin and what's going on outside the plane.

A small leak can cause slow depressurisation of the air in the cabin - if it's caught early enough, the pilot has time to make an emergency descent to a safe altitude. This is likely what happened in this situation.

But a change in cabin pressure, either slowly or suddenly, can still cause a number of health issues. A slow leak can lead to hypoxia symptoms such as nausea and headaches; and there's a more complex explanation for the blood leaking from ears.

As Chris Brennan-Jones, a paediatric audiologist, explains for The Conversation, the middle ear space is mostly airtight and protected by the eardrum.

However, air can get in and out of the ear through the channel that links the ear to the back of the throat, called the Eustachian tube.

"You have a small amount of air sealed in the middle ear space. And then there's all the rest of the air outside, in the atmosphere," says Brennan-Jones.

"Normally the air pressure inside the middle ear and in the atmosphere are very similar, or at least not different enough to cause you any trouble. As you increase in altitude the air pressure in the atmosphere decreases, making the air 'thinner', while the air pressure in the middle ear remains relatively unchanged."

This can put pressure on the eardrum, making things harder to hear, and causing the discomfort of a pre-popped ear.

When you have an extremely quick drop or increase in pressure, this can also cause eardrums to rupture, or cause something called ear barotrauma.

Symptoms include ear aches, and drainage from the ear that can be bloody or clear, along with potential nausea and vertigo.

Luckily, ruptured eardrums and barotrauma heal within a few weeks to months, with nearly no intervention necessary. So, despite this scary situation, it's unlikely anyone was seriously or permanently injured.

"In line with standard procedure, the crew deployed oxygen masks and initiated a controlled descent," Ryanair said in a statement.

Ryanair said the plane "landed normally and customers disembarked, where a small number received medical attention as a precaution."

But not everyone agrees. Passenger Conor Brennan told the Irish Times that "airport staff and Red Cross did their best to handle the situation, as Ryanair were nowhere to be seen."

"They really displayed a shocking lack of empathy for their customers, almost bordering on inhumane."

Still, according to the ABC, everyone was able to leave the hospital by Sunday morning.

It's a good reminder to actually listen to that safety briefing on the next plane you catch - so you know exactly what to do with the oxygen mask if it dangles in front of you all of a sudden.