"This October, journey into the depths of the sea and compete in AquaDome -- a FREE new arena located at the bottom of the ocean!
Two premium aquatic-themed Battle-Cars, Triton and Proteus, will also debut simultaneously with AquaDome (sold separately in the Showroom). Look for all this and more... soon!"
- Official Rocket League YT Channel
Patch notes: https://www.rocketleaguegame.com/news/patch-notes-v124/