Confirming months of rumors, Google today unveiled the company's first major foray into wireless service: Project Fi. According to the Google announcement, the service, as rumors had suggested, leans heavily on Wi-Fi while using the Sprint and T-Mobile networks as cellular backup, automatically detecting and switching the user to the best option at any given time.

For $20 a month users get unlimited talk, text, tethering and international roaming in 120+ countries.

After that, it's a flat $10 per each additional gigabyte of GB for cellular data whether in the U.S. or abroad (read: $10 for 1 GB, $20 for 2 GB, or $30 for 3 GB) up to 10 GB per month.

There's no contracts, and if Google Fiber is any indication, no sneaky fees (outside of the usual, government mandated variety). According to Google, you'll also get credited for any unused data at the end of the month.

"Since it's hard to predict your data usage, you'll get credit for the full value of your unused data," notes Google. "Let's say you go with 3GB for $30 and only use 1.4GB one month. You'll get $16 back, so you only pay for what you use."

There's more detail about the specific plans here, and the FAQ offers some additional detail regarding the way the network will operate. The catch: for now it's invite only, supporting just the Nexus 6 smartphone. Once Google expands what it's calling an "early access program," (read: open beta) the service should be made available for other handsets.