The Sui Sui no Mi is a Paramecia-class Devil Fruit that allows the user to swim through the ground or walls, making the user a Free-Swimming Human (遊泳自由人間,Yūei Jiyū Ningen?). It was eaten by Senor Pink.
This fruit's main strength is that it allows the user to swim in solid matter such as ground and walls as if it were water, the latter even allowing the user to swim upwards, seemingly disregarding gravity. It also allows the user to submerge into the ground to avoid colliding with it upon falling from a great height by performing a high dive.
It is noteworthy that like most devil fruits that can distort solid matter such as the Hira Hira no Mi or the Choki Choki no Mi, the distorted objects retain their physical properties such as weight and solidity in regard to other people than the user. Furthermore, after being swum through, the distorted matter "flows" back into place like a liquid would, leaving no trace of the distortion.
This fruit does not seem to have any specific weaknesses, aside from the standard Devil Fruit weaknesses where the user cannot use his power to swim on actual bodies of water. The fruit also has yet to show any offensive potential other than ambush and surprise.
Senor Pink has been seen using his power to swim in the ground and sneak up on his enemy from behind. It can also be used to allow him to escape, but he claimed that his pride does not allow him to do so. This may or may not be true, as he was protecting an incapacitated soldier behind him at the time. He can also swim upward of a building to gain height for increased momentum for a suplex move.
Pink ate the Sui Sui no Mi at no older than the age of 30, as he was seen using its powers to swim in the ground then.
Ironically, while the Sui Sui no Mi gives to the user the ability to swim through solid surfaces, due to the standard Devil Fruit weakness, the eater of this fruit remains unable to swim in water. This means that the Sui Sui no Mi is the only Devil Fruit shown so far that lets the user maintain (although in particular conditions) the ability to swim.