The Great Lake

The grounds of Hogwarts function partly as a nature reserve for magical creatures which have difficulty existing in Muggle-inhabited areas.

The lake is full of creatures that would make a Muggle naturalist swoon with delight - if terror did not seize them first. There are Grindylows (vicious little water demons), merpeople (of a hardy Scottish strain) and a giant squid, which is semi-domesticated and permits students to tickle its tentacles on sunny days, when it basks in the shallows.

Giant squid genuinely exist, though they are most mysterious creatures. Although their extraordinary bodies have been washed up all over the world, it was not until 2006 that a live giant squid was captured on film by Muggles. I strongly suspect them of having magical powers.

From Pottermore (www.pottermore.com)