Question: The Cirque de Gavarnie is compared to what oval amphitheatre built of concrete and sand is the largest amphitheatre ever built?

Context paragraphs:
Colosseum: The Colosseum or Coliseum ( ), also known as the Flavian Amphitheatre (Latin: "Amphitheatrum Flavium"; Italian: "Anfiteatro Flavio" ] or "Colosseo" ] ), is an oval amphitheatre in the centre of the city of Rome, Italy.  Built of concrete and sand, it is the largest amphitheatre ever built.  The Colosseum is situated just east of the Roman Forum.  Construction began under the emperor Vespasian in AD 72, and was completed in AD 80 under his successor and heir Titus.  Further modifications were made during the reign of Domitian (81–96).  These three emperors are known as the Flavian dynasty, and the amphitheatre was named in Latin for its association with their family name "(Flavius)".

Gavarnie Falls: The Gavarnie Falls (French: Grande Cascade de Gavarnie) is a tiered waterfall in France.  With its overall drop of 422 metres, it is the highest waterfall in mainland France.  The falls are situated in the Cirque de Gavarnie, near the village Gavarnie in the Hautes-Pyrénées.

Gavarnie: Gavarnie is a former commune in the Hautes-Pyrénées department in south-western France.  On 1 January 2016, it was merged into the new commune of Gavarnie-Gèdre.  Gavarnie is known for the Cirque de Gavarnie, and the Gavarnie Falls in it, part of the UNESCO World Heritage Site Pyrénées – Mont Perdu.

Irvine Meadows Amphitheatre (Irvine, California): Irvine Meadows Amphitheatre (formerly known as Verizon Wireless Amphitheatre) was an amphitheatre located in Irvine, California.  The establishment was built in 1980 by Irvine Meadows Partnership (with 4 local investors) and it opened in 1981.  It was the largest amphitheatre in Orange County, with 10,418 reserved seats and 5,667 on the lawn.  The architect was Gin Wong Associates of Los Angeles.  The unique design of this venue is that it's built on a hillside so the seats have a steep slope toward the stage.

Gave de Pau: The Gave de Pau is a river of south-western France.  It takes its name from the city Pau, through which it flows.  The river is 183 km long, and its source is at the Cirque de Gavarnie in the Pyrenees mountains.  The Gave de Pau joins the Gave d'Oloron in Peyrehorade to form the about 10 km long Gaves réunis (united Gaves), which is a left tributary of the Adour.  The "Gaves réunis" is often considered to be part of the "Gave de Pau".

Ecole Supérieure des Arts du Cirque: The École Supérieure des Arts du Cirque (ESAC) is a professional circus school located in Brussels, Belgium.  The Ecole Supérieure des Arts du Cirque de Bruxelles is one of very few colleges in the world to provide circus training leading to a diploma, along with the Centre National des Arts du Cirque de Châlons-en-Champagne in France, the Ecole Nationale de Cirque in Montreal, Canada, and the National Centre for Circus Arts in London, UK.

Cirque d'Estaubé: The Cirque d'Estaubé is a cirque in the central Pyrenees in the Pyrenees National Park, forming the frontier between France and Spain.  The cirque lies 4 km to the east of its better known neighbour, the Cirque de Gavarnie, which is accessible via the "Hourquette d'Alans" (2,430 m); the Marboré valley is reached via the "Brèche de Tuquerouye" ("Tuca Arroja") (2,666 m), where there is a small CAF hut.

Gavarnie ringlet: The Gavarnie ringlet ("Erebia gorgone") is a member of the Satyrinae subfamily of the Nymphalidae family.  It is a high-altitude butterfly found in France and Spain, notably at Cirque de Gavarnie in the Pyrenees National Park.

Cirque de Gavarnie: The Cirque de Gavarnie is a cirque in the central Pyrenees, in south-western France, close to the border of Spain.  It was described by Victor Hugo as "the Colosseum of nature" due to its enormous size and horseshoe shape causing a resemblance to an amphitheatre.  It is located within the commune of Gavarnie, the department of Hautes-Pyrénées, and the Pyrenees National Park.  Major features of the cirque are La Brèche de Roland (English: Roland's Pass) and the Gavarnie Falls.

Casque du Marboré: The Casque du Marboré, or Casque de Gavarnie or simply le Casque, is a pyrenean summit, culminating at 3006 m , located on the crest of three-thousanders in the Monte Perdido Range above Cirque de Gavarnie on the Franco-Spanish border.

Supporting paragraphs: Cirque de Gavarnie Colosseum
Answer: Colosseum