I am growing the cukes on a vertical slanted trellis and they are coming on like gangbusters for the first time. They are growing fast. I have a drip system and coastal hay mulch. They taste good in every way, but each slice in the middle of the cucumber has a small opening, looking like either a v or y. Other than that, they are great looking and tasting.
Sliced Cucumbers Have Opening
Certified GKH Gardening Expert
This is a very common problem. Can be caused by the watering, if the plants are watered then dry out, this can cause some hollowing of the cucumbers. To much Nitrogen can also cause this to happen. You can cut back on fertilizer if your using it.
If your cucumbers taste great, it is not a big factor.
My grandmother always used her imperfect cucumbers for relish! I sometimes do the same.
Here is a link for you with more information.