Travels Through Wormhole

In-Universe:
The U.S.S. Defiant makes it way to the Bajoran wormhole and enters, soon to be engulfed but the swirling colors and clouds.

Production:
The original studio model of the Defiant is first seen from below.

This scene appeared in these episodes:

EpisodeTitleTimestamp
3x01 The Search, Part 1 21:20 - 21:26
3x21 The Die is Cast 19:00 - 19:07
4x23 To The Death 3:40 - 3:49
6x06 Sacrifice of Angels 31:23 - 31:28