At {HEAD ~ each intersection} of {TAIL ~ a pair} of interconnected tunnels is a mirror mounted on the block 
of
At { HEAD ~ each intersection } of { TAIL ~ a pair } of interconnected tunnels is a mirror mounted on the block
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH