Stygiomedusa gigantea, commonly known as the giant phantom jelly is a part of the monotypic genus of deep sea jellyfish, Stygiomedusa. This is in the Ulmaridae family. It can't actually sting you as its—30-foot-long—”mouth arms” are just there to trap prey rather than sting them.

This ghostly giant is a rare sight.

MBARI's ROVs have logged thousands of dives, yet we have only seen this spectacular species nine times. The first specimen of the giant phantom jelly was collected in 1899, but it was not until 60 years later that scientists recognized this as a new species.

Had to add this on for perspective.