understructure (n.)

• base
• basement
• basis
• bed
• bedding
• bedrock
• bottom
• floor
• flooring
• footing
• foundation
• fundament
• fundamental
• ground
• grounds
• groundwork
• hardpan
• infrastructure
• pavement
• principle
• radical
• rudiment
• seat
• sill
• substratum
• substructure
• undercarriage