Save yourself an emergency call to a plumber
Tank water heaters are typically made out of steel with a glass lining. Overtime sediments in the water erode this lining and the steel tank corrodes until it punctures.
To extend the life of the water heater, manufactures include a sacrificial anode. This is a rod made out of another metal (Zinc, Magnesium, Aluminum) which is more susceptible to corrosion and protects the tank by letting itself be consumed first.
Once the anode is corroded, the tank is no longer protected and it will puncture.
You can inspect and replace the anode rod of your water heater to extend its life and avoid having to call a plumber in an emergency and the risk of water damage.
It’s a nice DIY project for the summer that takes no more than 30 minutes. Anode rods typically cost around $30 to $60.