contours developed from CartoDEM showing negative values

[mkcsrikanth]

Dear Bhuvanites,

I have downloaded CartoDem V2 R1 and extracted the contours, Some of you must have faced the situation given below:
1) What does negative values of Elevation/Contour mean.
2) How to get the positive values or convert negative or all the values to positive.
3) Does it mean that some places have elevational values below the mean elevational value? if so what would be the mean elevational value.

I would be thankful if Any help in this regard would be provided by bhuvanites

regards
srikanth