A puzzle cache in Essex.
THE CACHE IS NOT AT THE LISTED COORDINATES.
This diagram shows the first 200 entries in a sequence which goes on to infinity:
It defines a function L(n) which counts the number of yellow cells in the gap between the green cell numbered n and the next one above it.
It was first conjectured in 1808 that L(n) is greater than zero for all n greater than zero.
"Conjectured" means that most mathematicians believe that statement to be true, despite the fact that nobody, not its author, not Euler, Gauss, Ramanujan, not even Erdős, has succeeded in proving it or in finding a counterexample.
(If you can find a proof, or disprove it by finding a pair of green cells with no yellow ones between them, you're probably in line (if you're under 40!) for the Fields Medal 
)
The cache is at N L(259)° L(204).L(908)' E L(0)° L(112).L(935)'
