Tesla Towers are central to Radiation Island. The overall objective is to shut down all 19 of the towers, which are scattered around the three main islands. Each one is represented by a symbol showing the top of the tower, roughly a sphere on a pole with a ring underneath the sphere. This symbol is overlaid with a green tick when the tower has been deactivated.
Each one needs a unique 6 digit code to deactivate, all of which can be found in journal entries with a map of the location of the tower with its code scribbled next to it. One of these journal entries is found near the start, and from then on one can be found near the deactivation code input in the base of every tower. There appears to be only one "active" tower at a time, which is shown through an arc of electricity from the main tower to it, although the towers can be deactivated in any order, regardless of whether it is "arcing" or not.
When deactivated, each tower appears to retract into the ground, leaving only the spherical top and the ring underneath it showing.
POTENTIAL SPOILER ALERT!!
The codes for the towers, or at least those on the first island, are all six digit sections of pi (excluding the decimal place at the start), and are placed in order when viewed through the journal.