Diglett is a tiny, brown Pokémon that seems to be perpetually buried within the earth, leaving only its head visible. Its small stature makes it both the lightest and shortest Ground-type. It has beady black eyes and a large, round, pink nose. A Diglett in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team mentions that it has feet, though their size and appearance are unknown. It spends much of its time underground and has very thin skin. If Diglett is exposed to sunlight, its blood will heat up and cause it to grow weak.

Diglett lives in tunnels and caves under the earth, where it feeds on tree roots and vegetables. It is known to live in burrows left behind by Onix. It also sometimes lives in burrows near forests and has even been shown to live inside of trees. Diglett digs regularly through the earth at a shallow depth, leaving perfectly tilled soil in its wake. Some farmers welcome Diglett because it tills their fields and its droppings enrich the soil, but others consider it a pest for eating their vegetables.

Forms
Diglett has a regional form: Alolan Diglett.

In Alola, Diglett has developed three fine gold whiskers on its head from living in soil with high iron content. These whiskers function as sensors and allow it to monitor its surroundings without coming above ground, though it will still occasionally poke its head out to see outside. The whiskers can also be used for communication, wobbling around to relay mood or emotion. They stand straight up for anger, swing around for happiness, and droop for sadness. Needing to be able to dig through Alola's volcanic rock, Alolan Diglett is notably stronger than its non-Alolan counterparts. However, they are still very small, tying with Klefki and Meltan as the shortest Steel-type. Alolan Diglett and its evolution are the only known Pokémon that can have Tangling Hair as an Ability.

Despite Diglett having a strong resemblance to Wiglett from the Paldea region, the two Pokémon species are not biologically related, having entirely separate diets and habitats. Their resemblance to each other is said to be a coincidental result of adaptations to their respective environments.

Evolution
Diglett evolves into Dugtrio.
Alolan Diglett evolves into Alolan Dugtrio.

(Source: Bulbapedia)