This answer to a question states that:
.... construction left a large pit, now filled with water, that will be used for the Ariane 6 flame trench.
What is a flame trench, and what is its purpose?
The Flame Trench is the big hole used to manage the flame from the rocket initially. Here's a shot of the Space Shuttle's Flame Trench:
A closer look at the flame trench at LC-39A from the Space KSC blog:
Note that essentially it deflects the flame so that it won't damage the rocket. All flame trenches have similar objectives. My favorite example comes actually from a missile (RIM-161 Standard Missile 3 or SM-3) test, in which the flame trench essentially works like an exhast pipe, which is easily seen below: