Imagine you are walking through a busy city street when suddenly you see a flash of light out of the corner of your eye

Imagine you are walking through a busy city street when suddenly you see a flash of light out of the corner of your eye. You turn to see a strange, shimmering portal opening up in front of you. Curious, you cautiously step closer and peer inside. What do you see on the other side of the portal? How do you feel and what do you do next?