Merriam-Webster Collegiate Dictionary - undersurface

 I. noun  Date: 1733 underside  II. adjective  Date: circa 1934 existing or moving below the surface UNDERTAKE  verb  (undertook; ~n; -taking)  Date: 14th century  transitive verb  1. to take upon oneself ; set about ; attempt ~ a task ~ to learn to swim  2. to put oneself under obligation to perform; also to accept as a charge or responsibility the lawyer who undertook the case  3. guarantee, promise readily undertook that the letter should be securely conveyed — Sir Walter Scott  intransitive verb archaic to give surety or assume responsibility