Plop

[plɒp] or [plɑp]

(noun.) the noise of a rounded object dropping into a liquid without a splash.

(verb.) drop with the sound of something falling into water.

(verb.) drop something with a plopping sound.

(adv.) with a short hollow thud; 'plop came the ball down to the corner of the green'.

Edited by Gertrude--From WordNet

v.i. to plump into water.

Checked by Leon