If you keep an aquarium you might want to one day keep semi-aquatic creatures that require air. This habitat can be established underwater using a large glass jar, an extra aquarium air pump, a cheap air stone and some aquarium rocks or sand.
Instructables user infernisdiem demonstrates that all you need to do is have a prop for the back of the glass jar such as a rock. Slowly submerge the jar with the opening face up until you reach the bottom of the aquarium and tilt the mouth of the jar toward the centre of the tank. Add sand and/or aquarium rocks to the inside of the jar to keep it from moving.
Attach an air stone to the end of the air tube and feed that tube into the jar and attach the other end to your air pump and activate the pump. You may want to attach an air regulator to adjust how fast air will fill the jar.
This will create a cheap, DIY underwater dry zone for your not-entirely-aquatic pets.
Underwater Dry Zone for Fiddler Crabs [Instructables]