[ uhn-der-strey-tuh m, -strat-uh m ]
/ ˈʌn dərˌstreɪ təm, -ˌstræt əm /
noun, plural un·der·stra·ta [uhn-der-strey-tuh, -strat-uh] /ˈʌn dərˌstreɪ tə, -ˌstræt ə/, un·der·strat·ums.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for understratum
But there is an understratum of deep, calm sea, which the breath of the wildest tempest can never reach.The Life and Public Services of James A. Garfield|Emma Elizabeth Brown
Where the city is now built it is fine siliceous sand, supported by an understratum of clay, which is of varied colors.Petals Plucked from Sunny Climes|Sylvia Sunshine