Satellites and spacecraft don't last forever; eventually, their mission ends, or they run out of fuel. When the time comes, NASA tries to de-orbit them safely, as this prevents the decommissioned space equipment from adding to the space junk problem.
So that leaves the question: where do all those de-orbited spacecraft end up? Good question.
Given that NASA has tons of experience with de-orbiting things, they've got the process down pat. NASA can essentially choose precisely where spent spacecraft will land. The goal is to avoid any accidents, so landings are kept far away from populated areas, making the ocean an ideal place.
There just so happens to be a broad span of ocean about 3,000 miles to the East of New Zealand, and it's perfect for landing retired spacecraft. NASA calls this landing zone the 'spacecraft cemetery,' and more than 260 satellites have landed there to date.
NASA estimates that the chance of a human casualty occurring from landing a satellite at the spacecraft cemetery is below 0.0001%. Most smaller spacecraft burn up before ever reaching the surface, but larger ones sometimes fall into the ocean. From there, they'll either sink or float for recovery teams to snatch.