Question
I have a 1500 gallon pond and live in Michigan. Last week the liner bubbled up such that it forced out at least 60% of the water. We put rocks on it to try to push it back down but there is still so much air pushing the liner up that we still have less than 50% of the water space. We tried stepping on it to push it down but it's like there is a constant source of air pusing the liner up. Our fish are trapped in various nooks and crannies of the pond. Any ideas what the cause is, and how we can fix this problem? Thank you!

Answer
I started to make a section on my site about bubbles in liners.  See http://www.fishpondinfo.com/ponds/pcare2.htm#liner
Are you sure it's air under there and not water?  Air normally won't get under the liner unless your ground is belching gas.  It's usually water under the liner.  I'm afraid that you will need to remove the fish, other things, and liner.  Then, if it's air, it should push out.  If it's water, you'll need to divert it.  See that link for a tiny bit more.  

Have you tried to use your hands to move one of the "bubbles" to the edge and up out the top?  If so, did air or water come out?  Air will usually move along but, water, not so easily.

You may need to remove the water just to free the fish if they are truly trapped.  How old is your pond?  It's getting cold there, right?  I wonder if it's not too dire if you could wait to repair it during the spring.  Once the ground and surface water freezes, no new water should come up under the liner?  Has it rained a lot there lately?

It sounds like you're lost half the pond's volume.  I've read of people who lost almost the entire volume!  If there's not enough left to safely overwinter the fish, you may have to pull them out at this time of the year which is hard on them.  You'd have to set up an indoor pond or something in a place like the garage until spring.

I'm sorry for the problems.  Good luck!