https://www.gamespot.com/articles/pubg-savage-map-adding-underground-caves/1100-6458338/

In an effort to keep the game feeling fresh and interesting, PUBG developer PUBG Corp. has announced it will add new vehicles, more weapons, and an underground cave system to its new, smaller map. The studio briefly discussed all of this in a blog post today, outlining some of what's to come for the Codename: Savage map.

The underground cave is inspired by a real cave system--the Phraya Nakhon Cave--that exists outside of Bangkok, Thailand. You'll be able to parachute directly into the cave, and there are apparently "plenty methods of escape." Here's a rendering of what it may look like: