What best describes the undertow?

• A. Waves bounce off vertical surfaces
• B. Oscillation of water in enclosed basin
• C. Very steep slope, wave surges up beach
• D. Weak return flow beneath breaking waves

Answer: (D)

Undertow is the weak return flow moving seaward beneath breaking waves. After waves break and push water toward the shore, a portion of that water must come back out to sea, and it tends to travel along the bottom rather than high in the water column. That subsurface, offshore-directed flow is what undertow describes.

This is distinct from other phenomena: waves reflecting off a surface would bounce back, not create a shoreward-to-seaward return flow; a seiche is the oscillation of water in a basin; and a very steep beach with big surges is about run-up or breaker dynamics on the beach, not the subsurface return current. The key idea is the gentle, bottom-directed flow returning water to the ocean beneath the waves.