You did not add a country tag, but in most countries, a plot of land and any buildings built on it become one inseparable unit which usually can not be sold separately. If you sell one, you also have to sell the other. That means if your bank forces you into foreclosure, they will auction the house, the plot of land and the other things you build on and under it together. They then pay your debt from the proceeds of that auction and give you the rest.
You could argue that it is possible to relocate a "modular home" (with considerable investment) making it a movable entity which can be sold separately from the land it is built upon and transported away. But in reality this is rarely the case. In most cases, moving a home is more expensive than ordering a new one. And in this particular case it would be especially nonsensical, because there is very little value in a basement, well and septic tank without a house on top. Check your local laws about precendent cases about whether homes of your type are considered immovable real estate or not.
You might be able to avoid the foreclosure if you proactively approach your bank and ask them to renegotiate your loan conditions. A foreclosure is usually a last resort measure which is in nobodies interest. The bank misses out on future interest payments, might not even recover the full loan sum and has cost associated with the bureaucratic hassles involved in a foreclosure. It's a lose-lose situation for everyone. So the bank might be willing to decrease your monthly payments to prevent this from happening.
If what you claim is true and your property is worth a lot more than the debt you have, then it might be an option to sell your property, pay your debt from the proceeds and move into a smaller, cheaper home. You can usually achieve a far better price for real estate if you sell it through regular channels than through a foreclosure auction.
Another option would be leasehold. You sell your house (and only the house) where it stands to pay off the debt and take a monthly lease from the new owner for the land it is on. This model is pretty popular in some countries, but virtually unheard of in others (in some countries it's even illegal). If this is an option in your location, then you can use their lease to pay your rent for a cheaper apartment or house.
In any case, if you miss out mortgage payments, tell your bank what's up! They might be able to find a solution with you, but only if you communicate with them. If you just silently stop paying your mortgage, they will pile you under late interest and fees and get you into even more debt.
Edit from JoeTaxpayer - this is an example of a modular home. I add this to help make Philipp's point, the words "Modular Home" and "Mobile Home" are different, and in most cases, a modular home is simply not mobile. Modular refers to the method of construction, a high percent is done in a large factory, and shipped to job site for final assembly. Once completed, it's a house, period, no different than the house built one 2x6 at a time on site. If OP doesn't return to clarify, answers assuming a "mobile" home might need editing just to state that clearly up front as these would be very different answers.
An example of a modular home.