View of the topographic divide between the Martian highlands and lowlands. The mysterious deposits of the Medusae Fossae Formation are found in the lowlands along the divide. The radar sounder on ESA's Mars Express orbiter, MARSIS, has revealed echoes, in the image below, from the lowland plains buried by these mysterious deposits. The top arrows show the surface echo and the bottom arrows the subsurface echo of one of the hills made up of the Medusae Fossae Formation deposits. The subsurface echo is from reflections off the volcanic plains material beneath the Medusae Fossae Formation deposits.