Roblox: Official Save/Load Terrain API

-- Save Terrain

local savedTerrain = workspace.Terrain:CopyRegion(workspace.Terrain.MaxExtents)
savedTerrain.Name = "SavedTerrain"
savedTerrain.Parent = workspace

-- Load terrain

local savedTerrainData = workspace:findFirstChild("SavedTerrain")
if savedTerrainData then
    local pasteLocation = workspace.Terrain.MaxExtents.Min
    local pasteEmptyCells = false
    workspace.Terrain:Clear()
    workspace.Terrain:PasteRegion(savedTerrainData,pasteLocation,pasteEmptyCells)
end