What is another word for "subsoil"?

Roget's Thesaurus (1911) lists these synonyms for "subsoil": abdomen, abdominal, alluvial, alluvion, alluvium, ancon, ashore, backbone, bank, beach, belly, bosom, breast, cave, center, chalk, chersonese, circumscribe, clay, cledge, clod, clot, coast, coastal, coeliac, come, concavity, contents, continent, continental, crag, debark, deep, delta, derelict, domestic, dry, earth, earthy, endemic.