地 = 土 (earth/soil) on the left + 也 on the right for the sound. The 土 radical is itself a picture of a mound of earth on the ground. So 地 is, quite literally, 'the earth under your feet'.
天 and 地 — heaven and earth — are the great pair in Chinese thought: 天地 means 'the whole world / the universe'. 地 also reads 'de' (toneless) as the particle that turns an adjective into an adverb (慢慢地 = slowly). And 地图 means 'map' — a picture of the land.
A mound of soil (土) plus a sound part (也) — the EARTH / the GROUND.
天 (heaven, which you know) + 地 (earth) = 天地, 'the universe'. The pair appears in countless sayings, like 天地良心 ('heaven and earth as my conscience' — I swear it's true).