Caliginous

"Our help?" Katie asked. "If you're from the future why do you need our help?"
Thalia sighed "Because we are out manned, out gunned, and out of time."
"For what?" Annabeth asked.
"The war," The daughter of Zeus looked down. "We have been forced underground by our enemy, the gods are practically senile, and many of us are dying. If not dying from battles we are dying from wounds and sickness."

The Giant war has taken a wrong turn, the demigods have been forced under the earth to keep from being destroyed. Of course hiding in the earth wasn't their best idea since it is Gaea, but if they were to hide why not hide in the place no one would expect to look? The giants have destroyed everything the demigods have fought for so to take back what is rightfully theirs, the demigods have created a device to send them back through time. They decided to take them from their past and pull them into the future to even out the odds of the war once again.