Meaning of E-318 value taken by variable in Fortran

Question

I am performing some calculations in Fortran 90. A particular variable (declared as implicit double precision) is taking the value of something like -1.0420437349566899E-318. At first, I ignored it assuming that it was probably just zero (the value that the variable is supposed to take is also zero). But now I am wondering if E-318 indicates an error in my code. I am also asking this since this variable takes such values at various points in a loop and on occasions the variable also takes NaN as its value. This indicates that there is most certainly an error in my code but I have a different question. In general What is the maximum and minimum value that such variables can take in Fortran 90?