NASA Earthdata Search

Earthdata Search is NASA's unified discovery and download portal spanning more than 54,000 collections across all of EOSDIS's distributed archive centres — LP DAAC, GES DISC, NSIDC, PO.DAAC and the rest — covering the majority of the NASA-sourced datasets in this collection (SRTM, MODIS products, SMAP, GPM IMERG, MERRA-2, GRACE) through a single search interface with spatial/temporal filtering and direct download or cloud access.

For anyone not yet sure which specific NASA archive holds the data they need, Earthdata Search is the sensible starting point rather than guessing which DAAC to visit directly; a free Earthdata Login account is required to download.