Plutonium

View looking south at the excavated, and rebuilt, remains of the "New Plutonium " at Hierapolis.

Note the rectangular pool that today is filled with thermal water.  In the left (east) wall of the pool, there is an arched opening.  According to the excavator, noxious gases were (are?) emitted from this opening—and this was the "gate to hades"!  This arched way is flanked on both sides by a "colonnade" of pilasters in the Ionic order.

The pool was surrounded on three sides by stone benches on which worshipers would sit and observe the rituals taking place around and below them.  The benches are well preserved in the foreground (north), and on the left (east), but are largely missing on the far side (south) of the pool.

Above the pool, on the left, is a replica of the large statue of Pluto and two serpents, that was found here.

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In March 2013, Francesco D'Andria, professor of classic archaeology at the University of Salento and excavator at Hierapolis announced that he had excavated the well–known, to ancient authors, Plutonium at Hierapolis—known as the gate to the Underworld.

There is a very useful article in "seeker.com" that describes the discovery in detail.