I forgot I liked Brinstar Depths a good bit back in Melee, but I forgot it wasn't in Brawl or Smash 4 until I looked it up. The stage is based on Kraid's lair in the Metroid series. Though they took a lot of creative liberties as Kraid's lair doesn't look like this in any of the games for the most part. I missed this stage a lot.
Not exactly the most balanced designed stage. You can actually destroy a lot of the stage and make it separate.
Trust me you don't want to fall down. That Brinstar acid leaves quite a burn.
And it will rise up and fast, so be careful. Wait a minute... what does Isabelle expect to catch with that fishing rod? A Metroid?