The palindromic (reads the same both backwards and forwards) prime (only divisible by itself and one) number 1000000000000066600000000000001 is known as ‘Belphegor's Prime’ due to the central presence of the number 666, on both sides enclosed by thirteen zeros and a one.

The number contains superstitious elements that have given it its name: the number 666 at the heart of ‘Belphegor's Prime’ is widely associated as being the ‘Number of the Beast’, used in symbolism to represent one of the creatures in the Apocalypse or, more commonly, the Devil. This number is surrounded on either side by 13 zeros and is 31 digits in length (13 reversed), with 13 itself long feared superstitiously as an unlucky number (triskaidekaphobia).

Dr Cliff Pickover, author, inventor and mathematician, named the number as ‘Belphegor’s Prime’ but the number was first identified by an engineer, mathematician and prime number hunter Harvey Dubner, who determined that it was part of a sequence. The set of primes could be reached by altering the number of zeros on either side of the 666 to appropriate amounts. This is to say that the number 16,661, with no zeros at all, is also a prime number. Then the next time a similar palindromic number becomes prime is when there are 13 zeros ('Belphegor’s Prime'), then again when there are 42 zeros, then 506 zeros, 608 zeros and so on.

Another way to express the number is 10(13)666(13)1, which truncates the 13 zeros into modifiers in the parentheses.

More Info: en.wikipedia.org