Earlier this month, 18 stingrays and 3 sharks on exhibit at Michigan's John Ball Zoo died as a result of a mechanical failure in their unnatural confines.

The animals had been housed in a small outdoor lagoon, where guests were invited to reach in and touch them. To make the shallow pool livable, a life-support system of oxygen-supplying pumps was used to aerate the water. On July 8, however, after heavy rains caused those pumps to malfunction, all the animals were found to have suffocated to death.

Zoo officials called it a "heart-wrenching incident" - but critics argue that it was an accident waiting to happen.

"There are few exhibits deadlier than a touch tank," PETA wrote in a statement, "where sensitive aquatic animals are unable to escape a constant onslaught of groping hands that dirty and pollute the water with bacteria and where a single malfunction or mistake could suffocate and poison dozens of animals in one go."