While not in Antofagasta proper, but rather 22km north of the city, this enormous offshore arch – the centerpiece of a 31-hectare protected zone – is the most spectacular of the area's sights. Topped by marine sediments and supported by a sturdy volcanic base, the stack has been eroded into a natural arch by the stormy Pacific. It's situated on a short westbound lateral off the highway; there's a small Conaf-managed museum and cliff-top views over surrounding beaches.
To get there, take a Mejillones-bound bus operated by Megatur to the junction at La Portada. From there, you'll have to walk 2km west or catch an onward bus – these run only in the busy months of January and February.