A Martian gravity map shows the Tharsis volcanoes and surrounding flexure. The white areas in the center are higher-gravity regions produced by the massive Tharsis volcanoes, and the surrounding blue areas are lower-gravity regions that may be cracks in the crust (lithosphere). MIT/UMBC-CRESST/GSFC via NASA hide caption

toggle caption MIT/UMBC-CRESST/GSFC via NASA

This World Health Organization map shows the percent of the population vaccinated for measles in each country in 2013. Dark green is at least 90 percent. Light green is 80 to 89 percent. Orange is 50 to 79 percent. Red is less than 50 percent. Courtesy of WHO hide caption

toggle caption Courtesy of WHO