The Underwhere fountain
From the Super Mario Wiki, the Mario encyclopedia
The Underwhere fountain is a special fountain that spews out orange water found in The Underwhere, in the game Super Paper Mario. If Mario and co. step into the water, it restores all of Mario and Co.'s HP at no cost, making it very similar to an inn. It is located in the beginning area of The Underwhere, next to a few Shaydes.
If Mario and co. need more than 99 HP restored, instead of a number inside of the heart that signifies HP restoration, it is simply blank, while nothing happens at all if their HP is full.