This monstrous thing is the Ariane space rocket. It's an odd one to find on the Wild Coast, but it explains a lot.
With the final closure of the penal settlement in 1963, French Guiana took on a forlorn air. The economy was completely supported by France, and there seemed little future for the colony.
However, at about that time the European Space Agency was looking for a launch site. Among the places it considered were Madagascar, Algeria, and Australia. But easily the best candidate was French Guiana; it has space, and a huge expanse of ocean in front of it; it is on the equator (so satellites can be launched into geostationary orbit with 15% less fuel); it is stable and relatively secure.
From the ESA's viewpoint it has been a roaring success. These days, over half the world's commercial sattelites are launched form here, and it brings in around $1 billion a year.
For the locals, it's been a mixed blessing. Huge tracts of the forest had to be cleared of people, and roads were diverted. It has also created a new class within Guyana's society; the spacemen, or technocrats. Not surprisingly, the original Guyanais can be left feeling somewhat marginalised.
As for the rockets, they can be up, floating over Kenya within 20 minutes. Naturally, they are valuable machines, and are guarded by regiments from the French Foreign Legion. There is also a detachment of the Paris fire brigade on hand, in case of emergencies ...