What do I do with the mossy key?

[Omega Initiate]

I found a mossy key in a tunnel that is in the koi pond, you'll need a torch to find it. The tunnel was near one of the breakers that has to be flipped to open the hatch to get the fish bowl burgl chip. I'm not sure what the key is for; I'm hoping someone can enlighten me.

[BunderTurb]

There's a treasure chest near the T-Rex