Keurig's proprietary single-use plastic coffee pods. You do get reusable versions so you can put your own grounds in, though, which don't seem as bad.

 

Yeah, K-cups are pretty horrific. They use specially bonded foil and plastic to keep the coffee fresh and make sure it brews just right, but makes them completely unrecyclable. They're very popular because you need zero morning brain power to use them, so you can imagine how much waste that generates. And yeah, we used to have a normal espresso machine, which, as you say, fill it, lock it, push the button... Easy peasy. Now we use a big percolator, until I get sick of coffee... Then it's Yorkshire Tea time.