The Underground appears in a few episodes of BFDI, and it is usually accessed by digging downward or a heavy impact towards the ground.
In "Are You Smarter Than a Snowball?", Tennis Ball, Eraser, Coiny, and Woody are stuck underground. Tennis Ball wants to use the bathroom.
In "Get Digging", Fries orders Nickel, Bomby, Dora, Yellow Face and Rocky to get digging, and they dig deep underground. Fries says they can stop after losing the challenge.
In "Zeeky Boogy Doog", Golf Ball takes her team into her factory, which is located underground.
In "Getting Teardrop to Talk", Stapy and Marker dig down underground. Dora does the same and digs up a container of cyanide.
In "The Liar Ball You Don't Want", underground craters are utilized for the challenge.
In "This Episode Is About Basketball", Marker was playing Toss The Dirt and ended up underground.
In "Return of the Rocket Ship", X goes underground to try to find his treasure. The challenge is to go underground and try to find the treasure before him.
Tennis Ball, Coiny, Eraser, and Woody stuck in a hole.
Yellow Face digging a hole.
Marker and Stapy playing Toss the Dirt.
Dora digging a hole to make islands.