On Monday, millions of pilgrims begin arriving in Mina, a small valley five kilometers east of the Grand Mosque in Mecca, where they will spend several nights in what becomes, briefly, the largest temporary city on earth. The tents they sleep in bear little resemblance to what earlier generations encountered: Today’s structures sit on permanent steel frames, built from fire-resistant materials engineered to reflect heat, with air conditioning running throughout. The facilities remain in place year-round, maintained and upgraded between each season.

For the Tawaf, the ritual circumambulation of the Kaaba that pilgrims will perform throughout this week, Saudi Arabia undertook a vast rebuilding of the Mataf, the marble floor surrounding the Kaaba, expanding it across four floors and more than doubling the number of pilgrims who can be accommodated. The marble floor was engineered to stay cool regardless of the heat outside, which last year reached 51 degrees Celsius (124 degrees Fahrenheit).