Noticing a pattern yet? This is a stretch of the Hero Story that a lot of people tend to dislike... right after they get a whole bunch of Sonic in a row (Metal Harbor, Sonic vs Shadow, and Green Forest), they get welcomed to a series of levels that goes like this... Knuckles, Tails, Knuckles, Tails, Tails, and then Sonic. XD
Anyways, this stage from Sonic Adventure 2 Battle is called Aquatic Mine! This is a fairly simple stage for Knuckles since it's fairly small and closed off, unlike his larger, more difficult stages like Pumpkin Hill and Meteor Herd. The level has one neat feature where you can adjust the height of the water in the level and this allows new areas to be accessible depending on the height of the water.
I could've gone faster for this particular run, but eh, it's good enough in my book. =P
Enjoy!~