Diglett is perfectly adapted to its semi-subterranean lifestyle, never fully emerging from the ground. A Diglett reaches maturity only when all three of its heads reach the same developmental level, after which it is considered a Dugtrio. Most individuals reach this stage after two years.
Dugtrio are hermaphroditic, and breed only once in their lives. Both individuals then begin producing buds, which separate from the parent Dugtrio and become Digletts after ~two months. All Digletts produced by a single Dugtrio are genetically identical. Because of their extremely prolific nature, most people consider them pests. Dugtrio can live upwards of three years, during which they produce hundreds of thousands of offspring. However, captive Dugtrio that have been successfully prevented from breeding can easily live five times as long.
Everyone has their headcannons for what Diglett looks like below ground, and this is mine ;P