Katakura Burrow.
Katakura Burrow (硬殻穴, Katakura-ana, Literally meaning "Hard Shell Burrow") is a legendary place and the home of the Turtles and Tortoises summons, having an importance equalling Mount Myōboku. Even though it is rumored that Turtles do not hold a Sage Mode of their own, this question is still to be answered.
Katakura Burrow's Symbol.
Even though the name states it, it is unknown if the burrow is a cave, as it is unknown where it is exactly located. The symbol of the burrow is the kanji "首" (kubi), meaning "neck".