The observatory is the hub of the collection of equipment on the seafloor. Its three stories are home to several instruments and power sources for other equipment.
On the bottom floor, the power sources are protected in titanium cases. Like the glass balls that enclose the camera and float above the TAAM, the titanium casings must be perfectly round or they won't be able to withstand the pressure of 5 kilometers of water.
The Observatory's instruments live on the top floor. There are two identical instruments, called Acoustic Doppler Profilers (ADPs), with upward facing disks on top like a colony of large mushrooms. The mushroom tops release sound waves and receive their reflections off moving particles in the water to determine the direction and speed of the ocean's currents. The ADPs can measure currents up to 100 meters off the seafloor.
Also atop the Observatory is an LED light, which makes it possible for us to see things through the camera. Otherwise, ACO would be entirely dark because sunlight can't reach the bottom of the ocean.