As water heats up, it begins by getting rid of dissolved gas. This includes nitrogen and oxygen and carbon dioxide and argon (air). Normally water can hold onto these gases but as it heats it is no longer able to.
As it heats more, the water itself *becomes* a gas (steam) and forms even *more* bubbles.
In both cases, the reason for bubbles forming on the metal bottom is twofold:
1. The bubbles usually form where the water is hottest, which *tends* to be right at the metal which heats it
2. In chemistry, there is something known as a nucleation site. This is a place which contains some small irregularity, such as a dust speck or a microscopic scratch on a surface, and some interactions or reactions happen much more quickly at these sites. The liquid water contains very few nucleation sites, as it is more or less all the same, but the metal siding is covered in microscopic scratches and such and is absolutely *littered* with nucleation sites.
Latest Answers