Yes. We could do that today if we had a really good reason.
All of the high explosives we can reasonably work with have an explosive performance of less that 2x that of TNT per weight unit, so let's say they come up with something that's exactly 2. Now we have to come up with half a million tons of it.
In 2018, total U.S. consumption of explosives was 1.77 million metric tons, so we could definitely CREATE that much explosive.
Conveniently, the physical density also tends to be around 2g/ml, so you'd need a quarter of the volume of the same mass of water to hold it. We're talking 250,000 cubic meters. This isn't an immense volume. It's a third of the loaded displacement of your typical cargo ship. It's still a lot, though.
Part of the problem would be packing it all together in a formation that wouldn't set it off. Let's use a salt mine. The tunnels there are 30 meters tall an 40 meters wide. You'd need 208 meters of that kind of tunnel to hold your explosive. Not too hard.
I REALLY can't tell you how high you can stack RDX explosive. I'm not sure it's legal for me to disseminate that information, so let's just presume that we can pack the tunnels without setting it off.
Setting it off wouldn't be hard. If it's all connected, it'll act like a giant fuse. Setting it off all at once would be impossible, but we can get close. RDX explodes at around 7,000 meters per second, and we're talking a maximum distance of 208 meters. You'd want to position your tunnels so that the explosion started on the outside and worked its way in to get the maximum force in the center.
There you go. A megaton explosion. I wouldn't want to work on that project.