Waterproof ink is a form of ink that can be used in various pens and is intended to not be water-soluble and therefore not run or be ruined by exposure to water. Most ink is water-soluble and if exposed to even small amounts of water can become too blurry to read or completely wash off of a surface. Note however that this does not control how the surface written upon will behave when exposed to water. A waterproof ink that is used on normal paper may hold up to water exposure, but the paper it is written upon can still be destroyed by the water.
There are several forms of waterproof ink, and some regular inks can also be changed to make them waterproof. The most common form of waterproof ink is one that can be used to fill a fountain pen or comes in cartridges for use in ballpoint pens. This form of waterproof ink is typically only waterproof once it dries on the paper or other writing surface. While the ink is still liquid and when it has not yet dried on the surface, it can still be water-soluble and ruined, washed away, or destroyed by water or other fluids.
A less common and more specific form of waterproof ink is one that comes in a pressurized cartridge for use in so-called astronaut pens. These pens use the pressurized cartridge to allow writing upside down, underwater, and in the low gravity of outer space. This type of pen was developed by American inventor Paul Fisher in 1965 and used by the National Aeronautics and Space Administration (NASA) to counteract the effect of low gravity on standard ballpoint pens. These pens have ink that is not water-soluble and can be used to write on surfaces completely submerged in water without fear of running or ruin. The surface written on must, of course, still be able to survive exposure to water and so specially treated paper is recommended.
Many regular forms of ink can also be turned into waterproof ink by the addition of a small amount of glue to the ink. This amount can vary depending on the ink and typically requires a bit of experimentation to find the proper formula. Through this process, ink can be rendered waterproof from water-soluble, though typically only after the ink has dried. While this is an imprecise means of making waterproof ink, and usually will not be quite as reliable as inks purchased specifically to be waterproof, it can often be sufficient if no alternative is available.