Palantir Technologies, an ultra-secretive "big data" start-up in Silicon Valley, has raised a whopping $880 million, according to a filing from the U.S. Securities and Exchange Commission (SEC) on Wednesday.

The funding round values the company at $20 billion, making it the fourth most highly-valued tech start-up in the world behind Uber, Xiaomi and Airbnb.

This is the latest part of a funding round that began in July and raised $679.8 million earlier this month, according to the filing. The Palo Alto, California-based firm has now raised to over $2 billion.

The 12-year-old start-up uses so-called big data – massive data sets that can be analyzed by computers to reveal trends and hopefully lead to better decision-making – and applies it to areas ranging from defense to fraud.