A traditional marimba can be over two metres long. The wooden bars sit above hollow tubes called resonators, which make the sound louder and richer. The biggest bars on the left make low, deep notes. The smallest bars on the right make bright, high notes - like a piano laid flat.
It usually takes three or four people to play a Costa Rican marimba together. One person plays the high notes, one the middle, one the bass on the left end. They have to listen carefully to each other and keep time. It is a team sport as much as a musical one.
The wood is special. The best marimba bars are made from a tree called rosewood, which gives a warm, ringing sound. The wood is left to dry for years before a maker carves and tunes each bar by hand. Tuning is done by shaving tiny slivers off the underside until the note sounds exactly right.
Marimba music is played at almost every Costa Rican celebration: birthdays, weddings, town fiestas, school parties. There is even a national Marimba Day. Children sometimes learn to play it in school music lessons - starting with two mallets and a simple tune.