底 bottom

TAKE the speculum to the bottom of the cave! (it's got yo Mama's residue on it so we have to bury it before it infects the general populace)

to do XXX thoroughly ★★☆☆☆
徹 (do thoroughly) + 底 (bottom) + 的 (motivation / '-al') = 徹底的 (to do XXX thoroughly)

to do XXX thoroughly, exhaustively, comprehensively. Plus, isn't that the greatest pronunciation? tetetetetetetetete!!! That's the most wild sound since the past tense of 'warm': (atatakatta)

bottom of the ocean ☆☆☆☆☆
海 (ocean) + 底 (bottom) = 海底 (bottom of the ocean)