Adam Clarke on Revelation 6:9

The fifth seal - There is no animal nor any other being to introduce this seal, nor does there appear to be any new event predicted; but the whole is intended to comfort the followers of God under their persecutions, and to encourage them to bear up under their distresses.

I saw under the altar - A symbolical vision was exhibited, in which he saw an altar; and under it the souls of those who had been slain for the word of God - martyred for their attachment to Christianity, are represented as being newly slain as victims to idolatry and superstition. The altar is upon earth, not in heaven.