Migrants seeking a better life in Europe have died by the thousands in the Mediterranean Sea in recent years while fleeing poverty and bloodshed in Africa, the Middle East and Asia.

The precise figure of deaths is unknown. But the ill-documented toll is rising again Sunday as rescuers search for an estimated 700 passengers from a capsized boat north of Libya. The Italian Coast Guard has confirmed 24 deaths and 28 people rescued, but authorities expect the death toll to rise.

Authorities count only those bodies found in the sea, on shore, or aboard boats where migrants can die of thirst or exposure. Survivors often tell of fellow passengers who lost their lives at sea, but the bodies are never found.

Here is a list of the deadliest migrant boat events, based on bodies recovered or survivor accounts:

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Christmas 1996: 300 believed drowned in the waters between Malta and Sicily.

June 20, 2003: 50 corpses found, 160 migrants missing, 41 survivors in a shipwreck off Tunisia.

Oct. 20, 2003: At least 70 dead in waters off Sicily.

May 12, 2008: 50 dead off Sicily, including 47 who died aboard after suffering from exposure.

May 6, 2011: Boat carrying more than 600 migrants is shipwrecked off Libyan coast. Hundreds reported missing.

June 2, 2011: At least 270 missing from boat with 700 aboard sinks off Tunisian coast.

July 10, 2012: Motorized rubber dinghy deflates between Libya and tiny Sicilian island of Lampedusa; 54 die.

Dec. 14-15, 2012: At least 21 people die, six missing after boat sinks off Greek island of Lesbos.

Aug. 10, 2013: Six migrants drown, 94 rescued some 15 meters (50 feet) off a crowded swimming beach in Catania, Sicily.

Oct. 3, 2013: 366 people die, 155 survive after shipwreck off Lampedusa.

Jan. 20, 2014: 12 people, including nine children, drown when boat overturns near Greek island of Farmakonis.

Feb. 6, 2014: At least 15 migrants from sub-Saharan Africa die while swimming from Moroccan coast as they try to reach Spanish enclave of Ceuta. Police fire rubber bullets at the swimmers to force them back to Morocco.

Sept. 10, 2014: Some 500 Syrians, Palestinians, Egyptians and Sudanese drown after their boat is rammed by another boat of human traffickers off Malta.

Sept. 14, 2014: Libyan navy reports 26 rescued from a boat carrying 250 migrants off Libyan coast. About 200 missing and presumed dead.

Feb. 8-9, 2015: At least 29 die and 300 people reported missing after four boats become waterlogged in the frigid sea after leaving Libya.

April 12, 2015: Nine confirmed dead after boat capsizes off Libya. Some of the 144 survivors tell aid workers that about 400 aboard drowned.

April 15, 2015: Christian survivors from a boat of African migrants arriving in Palermo, Sicily, tell Italian authorities that Muslim passengers threw 12 migrants overboard to their deaths when fighting broke out on board.