a sea of glass, as clear as crystal: The most noticeable part of ancient theaters was the bright mosaic floor where the orchestra performed. This design helped people see the scene clearly (compare Revelation 15:2–4).
The four living beings represent all of creation.
covered with eyes: This phrase likely means they had knowledge or understanding. In the ancient world, figures were often covered with a feature to highlight a quality (for example, statues of Artemis were covered with breasts to emphasize fertility). Compare Ezekiel 1:18.