When something goes down below the surface of water, it sunks.
Sunks usually happen to things that are heavy or that are not supposed to be in the water.
Examples:
The rock sunks to the bottom of the lake.
The toy boat sunks in the bathtub.
The sunks treasure is hidden deep in the ocean.
The sandwich sunks to the bottom of the picnic basket.
The sunks ship is now part of the ocean floor.
The sunks anchor holds the boat in place.