Seven seas and one big issue.

When ships take on ballast, they take on more than water. Microscopic organisms, eggs, cysts and even the planktonic larvae of larger organisms are small enough to pass through the intakes and pumps.

If these organisms are transported from their native seas to other parts of the globe, they can devastate their new environment.
Besides outcompeting local species, they may risk the health and economy of the local population.

This is why IMO has identified invasive species in ballast water as one of the four greatest threats to the world’s oceans.