How did the creator of Strongest Battleground create this effect?

[jeanpaupaul]

Bubble Module - Distortion Effect

A simple module for making distortion effects. It can still render particles and glass but some visual errors might happen, I cannot fix those errors. Example Video: Assets Module: BubbleModule.rbxm (42.0 KB) Place: Place.rbxl (87.8 KB)

If I am not mistaken they use glass spheres to achieve this kind of effect.