I would suggest you see a real estate lawyer as soon as you can. You will need to look at your title records and other materials [plat maps and the like] for easements and a determination of who owned the properties when the wall was constructed. If the construction of the wall was created as an easement, you may not be the one to pay for repairs.
Also, the wall may have required a permit for its construction, so there might be some records in the local building office as to its construction. Difficult to know as you don't give any details as to the time of construction.
Next, you or the lawyer you retain, should seek the opinion of a licensed structural engineer as to the construction and repair of the wall, the cost and the effect of failure, i.e.: what will happen if the wall fails?
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