Mine passages are sealed remotely from the surface by drilling a borehole from the surface to intersect a mine passage. Fly ash is then pneumatically injected down the borehole in dilute phase at high velocity to form a partial seal obstructing more than 90% but less than 99% of the area of the mine passage. The emplaced fly ash displays the form of a conical pile with the flanking slopes curving concavely upward to the lips of a crater formed beneath the injection borehole. Sealing is then completed by injection of an expanding foam into the crater area.