Well done! You've found the tuba!
The tuba is the largest and lowest-pitched member of the brass family in an orchestra.
Usually there is just one tuba in an orchestra - mostly we hear the tuba playing basslines, though we do sometimes hear them as solo instruments!
Like most brass instruments, a tuba is made of metal tubing which is coiled into the tuba shape. If you unravelled the tubing it would be around 16 feet (over 5 metres!) long.