What did Lina and Doon see as they looked over the cliff in the cave?

In Chapter 20, Lina and Doon see the city of Ember as they look over the cliff into the cave.
Both children have to walk carefully through a tunnel to get to the edge of the cliff. The journey seems even more challenging because they have Poppy with them, Lina's baby sister. Effectively, the city of Ember is situated in a vast cavern at the bottom of the plunging cliff.
The children are amazed at their discovery and decide to send the city of Ember a message. So, they drop Doon's letter (which he originally wrote to Clary) down to the city. The letter explains how to get out of the city of Ember. Before Lina and Doon drop the letter down to Ember, they add a short message to it.
In the message, Lina and Doon tell the people of Ember that they have found a place that is "green . . . and very big." They instruct the people to follow the letter's instructions and to head for the river. Lina and Doon also tell the people to bring food with them and to join them as quickly as possible. Then, the children wrap the letter and a rock inside Doon's shirt. Lina then drops the entire package down into the city of Ember.