This is the back (at the east), of the Prado Museum. It is the newly built part (2007), with the back of the original building at your left, and the new Moneo Cube or Los Jerónimos building, to the right of the church with the same name.

A better view of the Moneo Cube:

Moneo Cube and San Jeronimo el Real
Moneo Cube and San Jeronimo el Real

The project of architect Rafael Moneo, for the expansion of the Prado Museum, was completed in the year 2007.  Moneo’s Cube, is a separate building that did not involve any substantial changes to the El Prado’s Villanueva Building, that has nearly 200 years of history.

The two buildings, are connected by a passage way as shown in the next photo:

Passageway between El Prado main building and Moneo's Cube
Passageway between El Prado main building and Moneo’s Cube

The connection between the two buildings is underground (on the Jerónimos Building or Cube), because it takes advantage and covers the gap between The Jeronimos Monastery (Calle Ruiz de Alarcon) and the Paseo del Prado.

This extension was finished on April 27, 2007, although the official opening took place six months later, on October 30, with a temporary exhibition of the most significant pieces of nineteenth century Spanish painting collection, which had been stored for ten years on the neaby Cason del Buen Retiro.