
The site adopted a JavaScript-based cryptocurrency mining service from Coin-hive, it uses about 20 to 30 percent of a visitor's CPU cycles to mine Monero coins. The Pirate Bay implemented this system on its search result and category listings, but not on the homepage of the torrent link pages.
The Pirate Bay is the first major website to test the JavaScript miner and it caused quite a lot of ruckus. Some users don't mind it but others are quite vocal that they don't want websites to steal their computing power.
In a blog post, the owners of The Pirate Bay explain this is a test to discover if the Monero JavaScript miner is capable of bringing in enough revenue to be able to get rid of all advertising. It's definitely an interesting concept and it will be interesting to see the results of this experiment.