Greta Thunberg and 11 other activists are approaching Gaza by sea aboard the aid vessel Madleen, aiming to challenge Israel’s naval blockade. Israel has warned it will act to stop the ship, raising fears of confrontation reminiscent of the 2010 Mavi Marmara incident.

The almost 20 months of war has torn Gaza apart, often displacing residents multiple times and killing more than 54,000 people, according to the Hamas-run Health Ministry. A U.S.-brokered ceasefire agreed in January collapsed in March as Israel enforced a blockade on all humanitarian aid into the strip.

There are a number of things that could cause the ship to turn around, like lack of food and water or problems with the engine.