MS Estonia, (previously had three other names, MS Viking Sally (1980–1990), MS Silja Star (–1991), and MS Wasa King (–1993)), was a cruiseferry built in 1980 at the German shipyard Meyer Werft in Papenburg. The ship's sinking in the Baltic Sea on September 28, 1994, claimed 852 lives and was one of the deadliest maritime disasters in the late 20th century.

The Estonia disaster occurred on September 28, 1994 between about 00:55 to 01:50 (UTC+2) as the ship was crossing the Baltic Sea, en route from Tallinn, Estonia, to Stockholm, Sweden. The Estonia was on a scheduled crossing with departure at 19:00 in the evening on September 27. She had been expected in Stockholm the next morning at about 09:30. She was carrying 989 passengers and crew.
According to the final disaster report the weather was rough, with a wind of 15 to 20 m/s (29–39 knots/33–45 mph), force 7–8 on the Beaufort scale and a significant wave height of 3 to 4 meters (10–13 ft) compared with the highest measured significant wave height in the Baltic Sea of 7.7 metres (25.3 ft). Esa Mäkelä, the captain of MS Silja Europa who was appointed on scene commander for the subsequent rescue effort, described the weather as 'normally bad', or like a typical autumn storm in the Baltic Sea. All scheduled passenger ferries were at sea.

The first sign of trouble onboard the Estonia was a strange sound of metal against metal heard around 01:00, when the ship was on the outskirts of the Turku archipelago; but an investigation of the bow visor showed no obvious damage. At about 01:15, the visor separated and the ship took on a heavy starboard list. At about 01:20 a weak female voice called 'Häire, häire, laeval on häire', the Estonian words for 'Alarm, alarm, there is alarm on the ship', over the public address system. Just a moment later an internal alarm for the crew was transmitted over the public address system. Soon after this the general lifeboat alarm was given. Soon the vessel lurched some 30 to 40 degrees to starboard, making it practically impossible to move about safely inside the ship. Doors and hallways became deadly pits. Those who were going to survive were already on-deck by then. A Mayday was communicated by the ship's crew at 01:22, but did not follow international formats. Due to loss of power, she could not give her position, which delayed rescue operations somewhat. The ship disappeared from the radar screens of other ships at around 01:50. Mariella arrived at the scene of the accident at 02:12; the first rescue helicopter arrived at 03:05.

Out of a total of 989 passengers and crew on board 137 were saved. The accident claimed 852 lives (501 Swedes, 280 Estonians, 23 Latvians, 10 Finns and 19 people of other nationalities), by drowning and hypothermia, (the water temperature was 10°C–11 °C/50–52 °F). 92 bodies were recovered. According to the official report, Estonia carried 63 inflatable rafts and 10 lifeboats. 142 of her 188 crew had undergone an IMO-approved safety training course and were proficient with survival craft whilst the vessel herself had recently taken part in a fire training exercise. It is believed the remaining 757 are inside the sunken wreck to this day.