  Assuming that spacetime tunnels -wormholes and ringholes- naturally exist in
the universe, we investigate the conditions making them embeddible in Friedmann
space, and the possible observable effects of these tunnels, including: lensing
and frequency-shifting of emitting sources, discontinuous change of background
temperature, broadening and intensity enhancement of spectral lines, so as a
dramatic increase of the luminosity of any object at the tunnel's throat.
