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Underground storage

Why Underground Cold Storage Works: 9 Notes from Running a Freezer in a Limestone Mine

By the operator ·

I run an underground freezer in the Valmeyer bluffs, an old limestone mine across the river from St. Louis. People are usually surprised it exists at all, so here are the basics of why it works, from where I sit.

A ground rule for this post: numbers here come from my own operation or are my own estimates. They aren’t industry averages. A different mine, building, or product mix will land somewhere else.

1. The mine has a long history

Limestone mining in these bluffs started more than 120 years ago, mostly for fertilizer. Parts of the mine were later used to farm mushrooms, from the late 1930s into the early 1980s. During the Cold War it was considered as a civilian defense shelter. Turning it into cold storage is just the next use.

2. The rock sits around 58 °F all year

The big advantage is ambient temperature. In my experience the mine holds about 58 °F year-round, whether it’s August or January outside. An above-ground building fights summer heat and winter cold. Underground, you start every day from the same steady baseline.

3. That steady baseline cuts energy use

Because you aren’t fighting outdoor swings, heating and cooling work less. You’ll often see underground space marketed with savings on heating and cooling energy commonly claimed at 40 to 50%. That’s a marketing range, not something I’ve measured. Your real number depends on your equipment, your setpoints, and how often doors open, but the direction is clear.

4. Freezers run from about −15 to 8 °F

The space isn’t just cool, it’s freezer-capable. Freezer setpoints vary by room, from about −15 °F to 8 °F. The colder rooms sit well below the 0 °F federal benchmark for frozen storage (USDA FSIS).

5. Frozen columns act like giant ice cubes

This is one of my favorite parts. The mine is held up by massive limestone columns. Once a freezer area has been cold long enough, those columns are frozen too, and they act like giant ice cubes. If the power goes out, all that cold rock helps hold the temperature. It doesn’t replace backup plans, monitoring, or a corrective-action process, but it buys you margin.

(If you need a log to record those checks, here’s a free cold storage temperature log template.)

6. The space is huge

In our mine the rooms are separated by solid stone pillars spaced roughly 50 to 60 ft apart, with an average roof span of about 45 ft between them. That’s a lot of open, clear volume to work with.

7. The ceilings are bolted deep

The roof isn’t left to chance. In our mine, ceilings get bolted 8 to 10 ft deep into the rock. If you’re evaluating any mine for storage, ask how the roof is supported and inspected.

8. Building underground costs less

You don’t have to build a structure. The walls, roof, and insulation are already there in the rock. It’s commonly claimed that building out underground costs about half what an equivalent building would. Treat that as a rough marketing figure, not a quote. It depends heavily on what you’re building out, and ground control isn’t free.

9. Storms and earthquakes are a different story underground

Under solid rock there are no tornadoes, no blizzards, and no roof to lose in a storm. Your product is shielded by rock. I’ve also been told by a geologist that in an earthquake, the mine moves as one solid piece of rock, which makes it among the safest places to be.

If you’re evaluating underground storage

Questions worth asking any underground facility, including mine:

I’ll go deeper on each of these in future posts. If you have a question you want answered, send it over.

More from the freezer: The Defrost Game and The Day From Hell.

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