Is this part of a septic tank

[Summer]

Me and my husband recently came across this property and it has an add on attached to it. Could anyone tell me if this is a septic tank or part of a septic tank? This would have been originally outside until they built the add on.

 

[Jamesplumbing06]

Never seen a cut out like that. Could be original cesspool or rain sistern used as a septic later then abandoned.