BT Suk 53b(1) :מי איכא דידע מהוא למכתב אחספא ומשדא למיא ומהדר תהומא לדוכתיה‏ : is there anyone who knows if it is permissible to write (the divine name) on a sherd and cast it into the water and the sea will go back to its place?
See the entire entry for thōm, thōmā n.m. abyss