Dementia researchers have developed a video game that could lead to the development of early diagnostic tests for the disease.
The way players navigate the 3D levels in Sea Hero Quest will be anonymously tracked and sent to the researchers.
Understanding how people navigate 3D environments is important because the skill is often one of the first lost by people who have dementia.
Researchers say the game could generate an unprecedented amount of data.
"We have never seen anything undertaken in dementia research at this scale before," said Hilary Evans, chief executive at Alzheimer's Research UK, one of the organisations involved.
"The largest spatial navigation study to date comprised fewer than 600 volunteers.
"Providing the research community with access to an open-source data set of this nature, at this scale, in such a short period of time is exactly the kind of innovation required to unlock the next breakthrough in dementia research."
The free game was developed by the charity, researchers from University College London and the University of East Anglia, with the backing of communications giant Deutsche Telekom.
Players follow a sailor's quest to revisit some of his father's memories, and battle fantastical sea creatures.
The routes they take will generate global "heat maps" that will show researchers how people generally explore 3D environments.
The ultimate goal is to develop new diagnostic tests that can detect when somebody's spatial navigation skills are failing.
Researchers from UCL said the game generated useful data about 150 times faster than lab-based experiments.
"In my research team, I could only test about 200 people a year, and that's working hard," Dr Hugo Spiers, from UCL, told the BBC.
"But last night I tested 200 people in one minute with this game."
Players can opt to reveal their gender, age and location to the researchers, although they can choose to take part completely anonymously.
"This project provides an unprecedented chance to study how many thousands of people from different countries and cultures navigate space," said Dr Spiers.
"It's a massive online citizen science experiment that will give us an idea of what is 'normal' through this game.
"If we tile that information together, we can get a sense of how dementia changes over a lifetime, and other demographic factors."
Dr Spiers stressed the game was not itself a test for dementia.
"If you're worried about your memory or any changes to your health, speak to your GP," he said.
The researchers plan to share the first results of their data collection in November.