substructure (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
- undercarriage
- understructure