Easiest solution: pick up old pallets at your local hardware store, warehouse, etc. They're literally built to be a platform for storage.

I agree concrete is the way to go and I also wonde if the space is actually moisture proof when it pours rain. We had a dirst vasement in a house we lived in I stored some stuff there and when we had a rain storm all my stuff got ruined , just enough moisture to make the dirt wet and all my boxes too.

Unless you're into root cellars, canning or something, I can't imagine what you'd want to "store" in a space with a dirt floor.

Get a floor, or don't STORE.

Pea gravel!

Thanks for the tips! I might wait until the rainy season starts to confirm the condition down there. Right now, though, I was considering coarsely leveling followed by a vapor barrier and just laying 1.5"-thick 12" concrete paver squares on them. I don't need it to be perfect as we won't be walking on it constantly.

Why do we start getting mold with storage? assume I'm storing everything in plastic bins.

What about constructing a deck like structure? It will be an inch or two off the dirt to allow for air circulation.

Simplest DIY deck, I suppose....place concrete blocks - the ones with the holes in them - at the corners and in regular intervals in the middle of the area and run decking lumber over them. Sort of like you do when you put slats on a bed frame.

I'm about 6" tall so i'm a little worried about reducing the height too much. Is there a reason why I want something elevated from the ground vs just dropping in concrete 12" pavers?

If you use wood, which is probably easier to do than the pavers which have to really be set into the ground so they don't shift, if the wood comes into contact with the dirt it will rot over time. Unless you have air circulation to allow it to dry out in between humid weather (rain, etc.)

I would suggest for now putting in some humidity gauges and perhaps even a couple dips or sinks in your dirt to see where any accumulations want to develop to plan for some sort of drainage.. to see just how much moisture your basement gets. You might want/need to run some dehumidifier action.

And also use pallets and such too- just get used to crawl space instead of walking basement for now. And don't store a lot there just yet., especially humidity sensitive stuff. This should be a bit temporary.

Then after your wet season and gauging what needs to be done to do some more proper flooring going on.. start planning.

If you have basement finishing or sealing companies in your area, call around and see how much finishing a basement is.

Vapor barrier used to be 6mil plastic. Now 10mil is recommended. Gota have it.

Wood doesn't touch dirt. Wood doesn't tough concrete. Concrete should be prevented from touching ground. Those are the rules.

Have you ever found something in the ground? It has an 'earthy' odour to it. An odour that never goes away. That odour is telling you that something is LIVING in the item. That is mould/mildew. Sometimes fungi...which LOVE cool dark areas with CONSTANT humidity. Mushrooms are the other word for fungi.

You are welcome to SEAL the plastic containers (air tight...seal must be broken each and every time you open the container). That gets difficult when you are working with large bins. Look at the air tight jars people use in their kitchens. They have that rubbery/plastic INSIDE rim and a big clasp that holds the lid firmly in place. Now the pasta/rice/cornflakes/sugar are kept fresh.

Without an air tight container, you will get earthy, musty, fungus-spore-covered stuff in NO TIME.

Even in ancient Egyptian tombs have a musty smell. The driest areas in the world STILL have moisture in the ground. California is no different. Without a properly poured slab, you will fight a battle that humans have been fighting since our inception (4mill. years ago and counting)

Go ahead and take a small plastic tote and place a couple of old shirts and some shoes in it. Put the lid on and then plunk it down on the dirt. Leave it there until sometime after X-mas. Pull up the tote. Check to see if the earth under the tote is DARKER. That is an indication of MOISTURE.

Now open up the tote in the house and stick your nose in it. Does it smell like fabric softener or does it smell like dirt? If it smells like dirt you will have mould and fungi spores present. It would take a pressure cooker or a fire to get rid of them. Either way, it doesn't bode well for the stuff stored in the tote.

Thanks all -- I will try out some of those experiments and wait until it starts to rain a bit to see if any water shows up. Thanks!

No need to wait for rain to do the tote test. Moisture is INHERENT in the ground. Even in deserts there is moisture just below the surface.