Ah, the largest open air concerts in Spain. As Detective Pux, I've seen my fair share of crowds and chaos, but these events take it to a whole new level. So, my dear Watson, grab your earplugs and let's dive into the world of Spanish music festivals.

First up, we have the infamous "FIB" or Festival Internacional de Benicàssim. This four-day festival is held in the small coastal town of Benicàssim and draws in crowds of over 50,000 music lovers each day. The lineup usually features a mix of indie, rock, and electronic acts, and the beachside location makes for a picturesque backdrop.

If you're a fan of electronic music, you might want to check out "Sónar", which takes place in both Barcelona and A Coruña. This festival has been going strong for over 25 years and has become one of the biggest electronic music events in the world. It features a mix of live shows, DJ sets, and workshops, so you can dance the night away while learning a thing or two.

For those who prefer a more alternative vibe, "Primavera Sound" might be right up your alley. This Barcelona-based festival has been around since 2001 and has grown into one of the most respected music events in Europe. The lineup usually features a mix of indie, rock, and experimental acts, and the festival's commitment to gender equality in its lineup has gained it widespread praise.

If you're looking for a true Spanish experience, you might want to check out "Arenal Sound" in Burriana, Valencia. This festival takes place right on the beach and has a lineup that leans heavily towards Spanish and Latin American artists. It's a great way to experience the local music scene and soak up some sun at the same time.

Finally, we have "Mad Cool" in Madrid. This relatively new festival (it began in 2016) has quickly become one of the biggest events in the country, with over 60,000 attendees each day. The lineup features a mix of international and Spanish acts, and the festival has been praised for its organization and sustainability efforts.

So, there you have it! A few of the biggest open air concerts in Spain. Of course, there are many more out there, but these are just a few of my personal favorites. Now, if you'll excuse me, I have to go buy some earplugs and a ticket to "FIB".