UPDATE 21ST NOVEMBER: The Endless Modes is live! It's free, remember, and comes in a patch with additional Frostpunk fixes.
"We've put a lot of effort to deliver you this mode," wrote developer 11 bit Studios. "We hope you're going to enjoy it immensely! Also, we tried to test both modes as thoroughly as possible - but being a really big piece of content that you can play, well, endlessly, some non-standard issues may occur in the late game. Worry not, though, 'cause our brave QA guys are always waiting for your feedback at firstname.lastname@example.org! Don't hesitate to contact us about any bugs/crashes."
ORIGINAL STORY 19TH NOVEMBER: Frostpunk is a fine, fine game. An exquisitely wrought city-builder in which you cling to survival through apocalyptically cold temperatures. It's thrilling, and if you can hold on through the colossal storm, you win. But your campaign ends there - you can play no longer.
It's a full-stop players have wanted removed since the game's release in April, and now - any day now according to developer 11 bit - it finally will be, in Endless mode.
Endless mode has two sub-modes: Serenity and Endurance. Serenity is for builders; it eases up on the challenges so you can concentrate on building cities. Endurance, on the other hand, is designed to test your mettle. Think of it as the High Score mode - how long can you survive?
To ensure you don't run out of things to do during long campaigns, 11 bit has introduced more events/points of interest outside your camp - the things you send scouts to discover on the world map. These will be periodically wiped clean by incoming storms, too, and replaced by a bevy of new things.
There's a new building, too: the Archives. Here you can collect and store information which looks like it's lifted from the 11 bit development floor - old plans for automatons and so on. But I'm not sure what use these have in the game beyond being interesting nuggets of knowledge for fans.
The exact release date for Endless mode isn't known, but in a post announcing a Frostpunk Steam discount today, 11 bit said "Endless mode free DLC is coming VERY soon".