Disclosing a (improperly) burried pool in Plano - compensation?

[Blake Lawrence]

We purchased a house as-is to flip.  After the fact we learned that they had buried a pool. Using google earth, I identified that the seller was the owner during the time the pool was filled in.  He did not disclose this anywhere on any documents.  It became an issue during a large storm last month. One of our laborers called to inform of us of smoke coming out of the ground.  What ended up being the problem was that ground water was getting vaporized by an electrical charge from the the power supply underground servicing the pool.  At that time we had already had found out there was a pool as the breaker box had a break labeled "pool".  We turned that off and the problem went away immediately.  We are installing a new breaker box and that power supply will obviously be removed.  However, on top of that electrical issue, the ground is sinking in very noticeably around the area where the pool was filled in, meaning the backyard may need additional work and the soil itself that was used may not hold grass or vegetation like the rest of the property.  

My questions are-  Did that need to be disclosed? If so, is there any documentation online that I can reference?

Did that work need to be permitted?  If so, will that be info that we can find publicly?

We have contacted the seller's agent and they responded back basically in all caps and exclamation points telling us no way they'd help, which leads me to believe something is wrong.

Thank you in advance from a rookie learning on the job.