Secret Hangar Location Needed for Entropy-sensitive Nano-bots

Premise

In this Earth-like world, a multi-billion dollar privately owned aerospace/defense corporation is devising a secret plan to manufacture a fleet of nano-bots. These nano-bot have swarm intelligence and appear as a silver metallic tempest once fully deployed and airborne. After years of simulations and prototyping, a final design was settled upon that will be best suited for their intended mission. The defense corporation has reached the following specifications below by design, and thus should be considered unalterable:

• Longevity: 5-10 years (depends on entropy of ambient environment during recharge)
• Durability: Water-resistant, debris-proof, shock-proof
• Max Speed: 800 mi/hr
• Operational Temperatures: -60C to 120C
• Material: made from a synthetic nano-tech substance that requires extremely rare raw inputs
• Production Rate: one-off, these will be the only series produced

• Power Supply Tech: [Classified]

• Flight Time: approximately 72 hours before needing to recharge

• Required Recharge Time: approximately 12 hours

Secret Hangar Location: unresolved

The hangar must be chosen carefully because the nano-bots are in a vulnerable state when they are recharging. They are vulnerable not only because they are immobilized, but also the nano-bots will lose longevity if they recharge in a place that has high ambient entropy. This is a weakness of the design that could not be avoided. The aim is that with the proper hangar, this flaw can be mitigated.

Analogy

Although it's not a perfect analogy, I want to make a quick comparison to how the US military uses hangars in the desert for its aircraft. In the desert there is less moisture in the air, meaning less corrosion and maintenance. Here is one such base:

At first glance it would seem that the desert has lower entropy than somewhere else, say if the military decided to keep the planes on the beach. (corrosion is mostly governed by kinetics, not so much thermodynamics, but just humor me here) However, holding a few things constant (volume of sand, type of sand, ect) the desert would actually have a higher entropy than the beach. This is because the desert has a hotter average temperature, and ceteris paribus, entropy for a simple system increases with temperature. One way to get the intuition behind this is:

$H-TS$

Where $H$ is enthalpy, $T$ is temperature, and $S$ is entropy. So while it's not a perfect analogy, we can see that the defense corporation as well as real life military both need to choose a hangar location that is best suited for their respective crafts.

Main Point: So instead of conventional planes and corrosion, we are instead dealing with nano-bots and entropy.

Question

What is the best natural location for a secret hangar for the nano-bots given that the defense corporation needs the hangar to be installed in a place with the lowest possible entropy: $S$?

Note: This natural site location is in addition to an artificial enclosure for the nano-bots. The defense corporation wants to minimize the entropy of the surrounding natural environment as a precautionary measure to protect their investment.

Success Metrics

• Minimize Entropy: Explain why/how your hangar location has the lowest ambient entropy and other relevant information, such as: open/closed system, temperature range, ect.
• Minimum Hangar Area: 100 square meters
• Terrain Manipulation: Digging, drilling, ect all fair game
• Geographic position: Can be located far away from defense corporation HQ
• Budget: $100,000,000
• Time Frame: constructed within 1 year
• How Secret?: at least not obvious to the naked eye, ideally not detected by xray/ultra sound imaging
• Power: The hangar itself doesn't power the nano-bots. No wattage will be needed. View the Power Supply Tech in the specs in the premise as a black box. The nano-bots charge themselves in a 12 hour period using a miraculous classified proprietary technology. The only thing we need to know about the power supply tech is that the bots risk decreasing their longevity if they recharge in the presence of a high entropy environment. So you might say the tech is miraculous till a point; there is a drawback. Lastly, this tech is 100% efficient; no heat is dissipated.
• Volume: The ideal volume in question of the natural environment surrounding the hangar is 20,000 cubic meters from the locus of the hangar. Meaning that, best case scenario, the natural environment will have low entropy for that much volume. Defense corporation is willing to be pragmatic though.
• Hangar Shape: Unresolved. Defense corporation may adjust the shape to fit the natural environment. If needed for your calculations, answerers may define their own hangar shape.
• Hangar Volume: 200 meters squared x 3 meters high = 600 square meters.
• Ingress/Egress: Humans will need to enter occasionally. Nothing larger than that though.

Reminder: We are only interested in a natural environment with low entropy. We cannot use equipment to manipulate the area around the hangar. The stipulation is for the mission, the outside natural environment must be natural. (you can rationalize this as staying clandestine or related issue)