@JaimeGallego That's the reason. In principle having multiple integral signs does make the notation clearer in the same way that something like $\mathrm{d}^2$ does, but it's so much more unwieldy to have several integral signs than to stick an exponent on the differential that most people just don't bother. You can usually tell how many integrals are being done by counting the $\mathrm{d}$'s anyway.
@Secret Many species of birds bully each other. Hence "pecking order".
@0celo7 That's absolute nonsense. My bookshelf has many books on analysis, with double integral signs all over the place. Moreover, in the particular example I was helping Abcd with, we weren't integrating with respect to area, so $dA$ would have been wrong. We were integrating twice with respect to time.
> Teams have turned bacterial cells into everything from photographic film to hemoglobin-producing blood substitutes to miniature hard drives, complete with data encryption.