The 55-day Maha or Great Kumbh Mela Hindu festival in Allahabad is expected to be the biggest religious gathering of humanity on Earth, with up to 100 million pilgrims bathing in the holy waters in January and February. The "mega mela" takes place every 12 years at the confluence of the Ganges and the Yamuna rivers - and the mythical Saraswati. The mudflats of the delta are transformed into a tent city, with the thronging masses visible from space (as the above satellite image from a smaller Kumbh Mela in 2007 shows).

That many people concentrated in one place should be viewable from space.

Or at least Google street view.