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The coronavirus has triggered the panicked to seek underground shelter and ride out the global pandemic ... but that -- plus the air they breathe -- may cost them MILLIONS.

TMZ spoke with some of the best bunker retailers in the biz, and they tell us business is usually slow this time of year but the coronavirus has business booming.

The folks over at Rising S -- their glitzy shelters range from 50k to a million bucks -- tell TMZ in just the last couple of weeks, they've had around 1,000 inquiries, with a bunch that resulted in sales for their super-custom underground shelters. Specifically, we're told Rising S was asked if their unique safe space can protect them from the ever-spreading coronavirus. The short answer? To a degree.

We're told Rising S -- whose backlog increased exponentially from 2 weeks to 2 months in the wake of the exploding crisis -- can supply, at an extra cost, non-perishable food and medical supplies. And, for those who plunk down a pretty penny, there are free perks -- customers will get a 5-gallon hand sanitizer (made by Rising S), masks, rubber gloves and bleach.

One of the most insane and lucrative purchase orders Rising S has recently received actually came from Japan ... when a customer bought around 1,000 air filtration systems 2 weeks ago for a whopping price tag of $3 million. But, most of Rising S's customers are in the U.S.

Over at Vivos Underground Survival Shelters, reps tell TMZ ... sales at the moment are up over 400%. Much so ... we're told potential customers are visiting Vivos Underground Survival Shelter's site in South Dakota and leasing bunkers.