solide, fiable, probant, pur, bon
It isn't dangerous, the ground is firm here, and very solid.
Ce n’est pas dangereux, le sol est ferme ici et très stable.
— Open Your Eyes (1997)
Have you seen any solid ground that would support your weight?
Avez-vous vu un sol solide capable de supporter votre poids ?
— Aguirre, the Wrath of God (1972)