Apple announced at the 2018 WWDC that Mac OS 10.14, due out this Fall, will be named Mojave. This macOS release adds support for new frameworks like HomeKit and Metal 2, and offers some nice (but not revolutionary) new features including: Dark mode, Gallery View, and Desktop stacks.
Since 2013 all macOS releases have been free to Mac owners, and named after landmarks in California. Initially it looked like Apple was sticking to a mountain theme with 10.9 Mavericks, 10.10 Yosemite, 10.11 El Capitan, 10.12 Sierra, and 10.13 High Sierra. Craig Federighi, Apple’s head of software engineering, hinted at WWDC that 10.14 Mojave’s dark mode was inspired by the beauty of the Mojave desert at night. While I’m sure that story was born in the marketing department, let’s hope Apple steers clear of Death Valley for their next macOS release name!