The Ice, Cloud and land Elevation-2 (ICESat-2) will add the third dimension to our study of Earth. ICESat-2 will measure the height of ice sheets, glaciers and sea ice with an unprecedented level of precision and detail. To do this, ICESat-2 will fly the most advanced laser altimeter NASA has ever launched. The laser fires 10,000 pulses of light a second. The instrument measures the time of flight of individual photons, down to Earth and back, within a billionth of a second. With this new technology, ICESat-2 will track annual ice sheet change to a fraction of a centimeter. Measuring the height of our polar regions will reveal ice mass changes and help us forecast how these changes will impact sea level and climate beyond the poles.