Data is stored in a network of  currently 900+ hosts around the globe offering their free hard disk space and getting compensated for that with Siacoins. Currently they are offering overall 3500TB of storage.



More data at: https://siastats.info/hosts_map.html and https://siastats.info/hosts_network.html

Each file you upload to Sia is split in chunks and distributed among 30 hosts with a redundancy of 3x.

The Reed-Solomon algorithm of data distribution ensures that even if only 10 of those hosts are online, your files will be still available for you to download.

As hosts risk Siacoins in what is called collateral, they are de-incentivized to drop the files.