What Lies Beneath
Unearthing Advanced
Compressed Air Energy Storage
More than 2,000 feet below the surface, Hydrostor’s patented A‑CAES technology stores energy in purpose‑built rock caverns, delivering reliability where the grid needs it most. On average, roughly a third of global landmass is suitable for A‑CAES caverns.
Underground Storage.
Above-Ground Benefits.
A-CAES facilities are deployed in hard rock geology that is widely available around the world. They do not rely on critical minerals or large amounts of land.
Key components for a standard 500 MW facility (hover to highlight):
Power & process facility (compressors, expanders, thermal storage system, balance of plant equipment)
Closed‑loop, non‑consumptive water reservoir
Take a Closer Look at
Our Cavern Design
Caverns are excavated deep into bedrock using a room and pillar layout, which is extensively used in underground mining for its flexibility and safety, ensuring cavern integrity over a 50+ year operating life. However, in other ways, A‑CAES caverns differ fundamentally from mining and tunneling infrastructure, which demand fixed horizontal alignment. A‑CAES cavern layouts are inherently flexible and can adapt to site conditions encountered underground.
Cavern Model
FOR ILLUSTRATIVE PURPOSES – MODEL HAS REDUCED DETAIL
Cavern Depth
2,000-2,500 feet below ground
Height of 1.5 Empire State Buildings
Water Volume (One-Time Fill)
~150m3 per MWh
Comparable to filling ~50 Olympic swimming pools (30ft depth) for 500 MW / 8 hour facility
Cavern Pressure
~60–75 bar
Scuba diving tanks operate at 200–300 bar
How Energy is Stored and Released
A-CAES acts as a giant air battery storing potential energy in the volume of compressed air within the cavern.
To charge the system, compressed air is injected into the cavern by large air compressors; to discharge the system, compressed air is removed from the cavern and used to spin a turbine-generator.
To increase the energy density of the cavern and improve the operating efficiency, the cavern is flooded with water and connected to a water reservoir on the ground surface – referred to as hydrostatic compensation.
The cavern can operate at a fixed pressure with the weight of the water column acting as a massive underground piston. As compressed air is injected into the cavern, water is pushed out of the cavern and into the water reservoir – lifting the water piston. As compressed air is removed from the cavern, water re-floods the space, pushing air to the surface – lowering the water piston.
0% Charged
The cavern is filled
with water
100% Charged
The cavern is filled
with compressed air
How We Build
Our Caverns
Each project deploys hundreds of skilled workers from the mining and oil & gas industries, unlocking a highly scalable and repeatable project design. Our process draws on decades of underground construction expertise and employs well-established cavern and shaft construction methods used globally in hydrocarbon storage and mining, drawing on hundreds of proven precedents.
Conventionally Sink Shaft #1
Using controlled detonations from the surface to cavern depth (0ft to 2000ft), the initial shaft down to the cavern is made. This becomes the air shaft, carrying compressed air during plant operations. The final shaft is 4ft in diameter.
Excavate Cavern
Mining equipment is lowered through the air shaft, where it is then reassembled underground and used to excavate the cavern. Excavated rock is removed via shaft #1.
Bore Shaft #2
The second shaft is made using a “raise bore process” (mechanical method used to excavate vertical shafts without explosives) all the way from the cavern depth at 2000 ft back to the surface. This becomes the water shaft, which is lined and cemented and extends into a sump below the cavern floor to maintain a water seal. The final water shaft is 8ft in diameter.
Geology Criteria
Each site undergoes rigorous geological evaluation to ensure safe, reliable, and long‑term operation. These include field and downhole characterizations (in-situ packer permeability tests, geotechnical and geophysical logs) and lab tests (mechanical and engineering parameters).
Key criteria include rock formations that are:
1. Naturally Strong
Capable of supporting large underground excavation
2. Impermeable
Prevents air or water leakage through the cells of the rock
3. Insoluble
Ensuring long-term stability when in contact with water
Using the Air as a Store Vessel
To build secure underground caverns, we look for ‘hard rock’ environments. This means we target igneous and metamorphic formations rather than softer soils or sands.
These are the same types of rock used for granite countertops; they are incredibly dense and resistant to pressure, making them the ideal natural container for our underground facilities.
Core Samples From Our Projects
Common Questions
Are underground rock caverns proven?
Yes. Deep underground caverns have been used safely for decades to store industrial products and energy resources worldwide. Mining and underground storage experience demonstrates the long‑term stability of properly designed rock caverns.
Who builds Hydrostor’s caverns and systems?
Hydrostor partners with global leaders in underground construction and turbomachinery—firms with decades of experience delivering large-scale energy and subsurface infrastructure.
Can seismic activity affect A-CAES caverns?
A-CAES caverns are located deep underground in competent bedrock, similar to other underground mining and storage facilities that operate safely in seismic regions. Site selection and design explicitly account for regional seismic conditions.
Does A-CAES affect groundwater or aquifers?
No. A-CAES operates as a closed-loop system with double-lined air/water shafts and is designed to avoid interaction with groundwater or aquifer resources.
Why the World Needs Long Duration Energy Storage
Hear directly from our team
Learn More About A-CAES
Explore the charge and discharge process and its benefits.