This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. A floor beneath a main basement of a building.

n. A basement located beneath another basement

n. A basement lower than the main basement of a building. In the largest modern buildings there are sometimes three or four of these, one below another.

"These included the Manhattan Project and housing the 'Arsenal' robots in a hidden subbasement in his mansion."

marines'    chancellery    half-completed    ground-level    eighty-sixth