Inside the U.S. Botanic Garden on a nippy Saturday afternoon, it wasn’t the humidity or the greenery that made the indoor gardens feel otherworldly — it was the shimmering ambient music being played on a fleet of boomboxes.

A crowd of nearly 300 had assembled for a “boombox walk” hosted by local experimental pop duo Bluebrain, who had composed music specifically for Saturday’s event.

Here’s how it worked: Participants who brought a boombox were given either a CD or cassette, while others toting hand-held devices had downloaded the group’s music before showing up. After a countdown from 10, everyone hit play simultaneously and marched around the Botanic Garden’s teeming flora for the next 20 minutes, while Bluebrain’s dreamlike composition echoed from dozens upon dozens of speakers.

Brothers Hays and Ryan Holladay of Bluebrain have focused on site-specific compositions such as this one for the past few years, but the boombox walk concept comes from composer Phil Kline, who has staged similar happenings in New York. After boombox walks hosted by the brothers in Dupont Circle and on the Mall, Saturday’s walk was Bluebrain’s best-attended. Unfortunately, it was also its last. The duo plans to spend 2012 tackling new projects.

Watch video from Saturday’s walk, shot by the Post’s A.J. Chavar, above.