Scientists are getting more worried that a volcano in Indonesia is going to erupt soon.

Mount Merapi has been threatening to blow for weeks, but on Thursday the largest cloud of gas so far burst out from it.

The volcano has been on the highest alert status for almost a month now, meaning it is ready to erupt.

Officials said more than 18,000 people have left their homes near the volcano, although some have stayed behind.

The ground near volcanoes is very good for growing crops in, and some villagers don't want to leave their farms until they have to.

It's feared thousands of villages could be covered in ash and lava if Merapi does erupt.

Indonesia has at least 129 active volcanoes, including Mount Merapi which killed 60 people in 1994. In 1930 a further 1,300 people were killed when it erupted.

The volcano is close to the area that was hit with a huge earthquake on 27 May, which killed 5,800 people and left more than 420,000 people homeless.