The Tower of Light is both, seen and mentioned in H20's season three finale "Graduation ." It is actually made of the Moon Pool sea water and was originally made by the Irish mermaid Eva (whom the comet was named after) in 1740; and is again made by Bella, Cleo, and Rikki in the show. The tower was used to stop a comet from hitting the earth in both situations. It is also possible that this is a power and an ability, even though it has been created in two totally separate locations: Mako Island and the Sea Caves of Ireland , which is the very same sea cave where Bella became a mermaid at the age of nine years old.
How It Was Made
The Tower of Light is seemingly a tower of water made from the Moon Pool. Just like controlling the weather, Cleo, Rikki, and Bella had devised a plan to recreate the Tower of Life using their mermaid powers. First, Cleo used her hydrokinetic power to form water in the shape of a pillar. Next, Rikki used her hydro-thermokinetic power to heat up and create thermal pressure from inside the water pillar, causing it to build up electricity. Lastly, Bella's gelidkinetic powers had created shape to control the water and formed a pillar -- in combination with the thermal structure inside it. As it formed more pillars circling around them, the girls' powers struggled to intake more and more pressure of the Moon Pool to create a vortex. Finally, Bella's other two powers of substicankinetic and mecokinetic powers had hardened the water, creating a vortex of intense pressure of water and was sent off as "a tower of light" effect. That tower was sent off towards the meteor, and knocks it back to the other direction.