Below the ground; underground space (e.g., subway, basement).
駅の地下で待ち合わせをしましょう。
Let's meet in the station underground.
古い建物の地下に貯蔵庫があった。
There was a storage room in the basement of the old building.
地下水位が上がって庭がぬかるんだ。
The water table rose and the garden became muddy.
The world of the dead; the grave (religious or figurative sense).
古代の人々は地下を死者の世界と考えた。
Ancient people thought of the underworld as the realm of the dead.
彼の墓はあの静かな地下にある。
His grave lies in that quiet place below ground.
Underground (activities); secret or illegal operations—e.g., underground organizations or black markets.
その都市には違法な地下組織が潜んでいると噂される。
It's rumored that illegal underground organizations lurk in that city.
戦時中、抵抗運動は地下で活動していた。
During the war, the resistance operated underground.
地下経済の一部は税の網をかいくぐる取引だ。
Part of the underground economy consists of transactions that evade taxation.