Trough. The crest is the highest point of the wave while the trough is the lowest point.
hollow
skreis
core, bore, hole,
jigsaw The 6 letter word ending with w is hollow, sorrow, pillow, etc.
hollow, ornate, turgid, flashy
calm
undulate
amain
IDEAS
a hollow intestine
That tree is hollow. Do you have a hollow leg? I'm going to hollow out a hole.
The word hollow is an adjective. It means to have an empty space inside.