Where I live (NJ, USA) it's illegal to bury a pool. If the pool was filled with soil and hidden it's possible you would have recourse to get the person who did it to pay for remediation. Unlikely but possible. However if the pergola structure was built over the pool like a platform, it may be legit.
The correct way to permanently remove a concrete pool, where I live, is to break the concrete up into pieces no bigger than 8 inches in any direction. For something that size you're looking at a jackhammer attachment to a sit-in machine of some kind. I think I saw a specialized one with an arm that had both a jackhammer and back-hoe on it so the pieces could be moved to their desired resting place..
You start by breaking up the floor of the deep end, then do the shallow end and push the pieces on top of the deep end. Then you break up the walls and make a layer on top, then the deck goes in last. All in small pieces.
All biodegradable material MUST be removed and not buried in the hole, as must metal, plastic, and anything other than concrete and dirt. Leaving beams or logs in the hole will cause problems later (and fail inspection).
After doing all that you get an inspection, then add clean fill if necessary (free from a local construction site) and finish with landscapers cloth and top soil. Expect some subsidence and to re-apply top soil after a couple of years. You may not need much fill but in a more typical pool with a real deep end you would. I'm not sure if the cloth goes between rock and fill or between fill and top soil. If someone adds that to the comments I'll correct this.
If your house is rated (for local taxes) as having a pool, removing it will reduce your tax bill and makes it worth the hassle of pulling permits and doing it properly.
Note there may be live buried wires in the area beneath and around the pool. Get to know the installation before you proceed, especially any surrounding lighting or pool machinery.