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I've searched this forum and found a few diy ideas, but I was wondering if anyone has found a commercially available cave structure large enough for a full frown RES. It seems that anything that companies make for turtles is for baby turtles. It makes me sad because I know people are just getting rid of turtles when they get too big.(possibly because it's hard to find things for big turtles?) Has anyone come across ANY big underwater caves? Thanks.

I have not seen any that large and since females can be up to 12" SCL, I'm not sure if you want something like that in your tank. It'll be a good opportunity to create something though.

Unfortunately, I think you're right. Turtles are 'cute' to most people while they're small, but lose the attraction as they grow. The sad reality of it is that 4" turtles are the most sold by pet stores and so they want to sell items that fit a 4" turtle.

However, on the DIY route:
I think a 6 quart oil drain pan would be a perfect starting point. A hot knife or Dremel tool should make short work of cutting opening(s), then you can decorate with rock, plants, sand, etc to fit your decor.
http://www.coxhardware.com/p-3011-round ... quart.aspx

I made a large open cave from limestone and JB Weld "waterweld" epoxy.