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I designed this sea-mine last month as a challenge for our pet crayfish Crusty. He was always climbing everything (air pipes, thermometer, plants) and is extremely driven by food.

Searching the internet I found similar designs with the concept I was looking for but none that could print support-free and had a good solution to keep the base at the bottom while the mine has good floatation (Both PLA and PETG sink over time).

That's why I designed my own. It consists of two halves that can fit a standard table tennis ball in order to float well. It works without, but might sink over time.

The print-in-place chain has custom supports, so no support or brim needed. The chain gets sandwiched between the halves and there are three separate pins that twist-lock these halves together, requiring no glue or fasteners. They need to snap in; if they are too loose or too tight you can try one of the under- or oversized pins.

Because it encapsulates a table tennis ball there is a lot of floatation, allowing even heavier crustaceans to climb to the top. The base is extra large but a couple of stones are already enough to anchor it to the bottom.

There are also four holes where an optional screw can be inserted from the inside out to which food can be attached. 

It took some training to get Crusty on the mine the first time (we stuck some corn inside the chain) but once he has been up there, every time he walks by and sees some color he'll climb right up to check it out!