Tunnel Portals are built at the entrances to tunnels. They hold back rocks and dirt from tunnel entrances and are often used in combination with Retaining Walls. Easy to color with Earth Colors Liquid Pigment.Outside: 9 1/16 in W x 8 3/8 in H (23 cm x 21.2 cm) / Inside: 4 5/16 in W x 6 1/4 in H (10.9 cm x 15.8 cm).