Underneath rhymes with:
beneath, beneath, bequeath, faeth, gaeth, heath, keath, keeth, keith, leath, leeth, leith, meath, paeth, reith, rujith, seith, sheath, sleeth, spaeth, teeth, vaeth, veith, weith, wreath
Underneath sounds like:
unadorned, underhanded, undermanned, undermined, underwent, undetermined, undreamed, untrained, unturned