As Andrea explains on Making it Lovely:
I chose succulents as the focal point in the terrariums because even though they are really dry, desert dwellers, their shape and color also remind me of algae and seaweed you would find in the ocean. I selected tall, spiny looking succulents for the arrangements, with a few grass-like ground covers as well, to represent algae. The glass globes were picked up at a local craft store and although they are technically meant to be votive holders, their shape remind me of old glass fishing balls. In addition you will need some coarse sand and cactus soil.
Find the full instructions at Making it Lovely.
(Images: Andrea Howe)