Starting in curl 7.58.0
If the total size is unknown, it will now instead display a small space ship flying across the line, back and forth – and it will only move as long as there is data being transferred. If it stalls, the little ship stops.
Daniel calls this new progress bar style "useless", but we always love seeing people inject fun and whimsy in to their open source projects, even at curl's state of maturity.
Curious what it looks like? There's a sample video on YouTube.