Hundreds of people were feared dead after a ship crowded with migrants capsized and sank in the Mediterranean Sea off Libya on April 19, as authorities described a grisly scene of bodies floating and submerging in the warm waters.

The fatal shipwreck may prove to be the Mediterranean's deadliest migrant disaster ever and is only the latest tragedy in Europe's migration crisis.

Here's what you should know about the illegal migrant voyages:

How many people have died?

The Mediterranean crossing from Africa to Europe has been described as "the most lethal route in the world" by the United Nations Commissioner for Refugees (UNCHR). Last year, a record 3,419 migrants lost their lives crossing the sea on rickety boats. This year – before the latest disaster occurred – an estimated 900 migrants had died, compared with 90 during the same period a year ago.

Libya is a popular starting point for many journeys, with human traffickers exploiting the country's power vacuum and increasing lawlessness. The relatively short distance to the Italian coast of Lampedusa also encourages more people to risk the journey.

Who are the illegal migrants?

Warmer spring weather usually unleashes a torrent of smuggler boats, bearing migrants and refugees from the Middle East and Africa, often fleeing war and poverty for a foothold in Europe. These people come from countries like Libya, Nigeria, Eritrea, Syria, Somalia and Egypt.

An estimated 218,000 people crossed the Mediterranean Sea last year. A total of 31,500 people have made the crossing to Italy and Greece from North Africa so far this year. In the week of April 10-17 this year, 13,500 migrants reached Italy. 

Why is Italy such a popular destination?

Before Italy, Greece was long thought of as the easiest way for potential refugees getting into the EU as its borders were very porous – the government could not afford to patrol the borders between Greece and Turkey. But in late 2012, the Greek government erected a barbed wire fence along the Greek-Turkish land border that kept out potential asylum seekers. Since then, the flow of refugee seekers has moved from land to sea, to different Greek islands in the Aegean, and eventually to Italy and Malta.

Plans to seal the Turkey-Bulgaria border will likely have a similar impact on the behaviour of desperate migrants fleeing from war and famine.

Why is the journey dangerous?

Factors such as bad weather, overcrowded vessels, sickness, hunger and thirst often contribute to the risks involved. Sometimes the vessels set sail without enough fuel, and the captains and crews abandon the ships, leaving passengers to fend for themselves.

What's been done by governments?

For the past several years, Italy has been in the vanguard of rescue efforts, with its naval and coast guard ships rescuing more than 130,000 people last year in a widely praised programme known as Mare Nostrum. The programme started in October 2013 as an emergency response to a shipwreck that killed more than 360 people near Lampedusa.

But Mare Nostrum was phased out last autumn after Italy's EU partners refused to help with the cost of running it amid a bitter row over whether the life-saving operation was unintentionally encouraging migrants and asylum-seekers to attempt the perilous crossing from North Africa to Europe. It was replaced by the European-led Triton, which has fewer ships and a less defined mandate.

António Guterres, the UN high commissioner for refugees, has called on Europe to expand its rescue and patrol programme, as well as legal avenues for migration to Europe so that people do not have to risk their lives at sea.

"It also points to the need for a comprehensive European approach to address the root causes that drive so many people to this tragic end," Guterres said. "I hope the EU will rise to the occasion, fully assuming a decisive role to prevent future such tragedies."

(SOURCE: NEW YORK TIMES, THE GUARDIAN, CNN, BBC)