ggplot(mountain) +
aes(x = x) +
aes(y = y) +
coord_3d() +
aes(z = 0) +
geom_surface_3d() +
aes(fill = z) +
scale_fill_viridis_c() +
aes(z = z) +
guides(fill = guide_colorbar_3d()) +
theme_light()