Index: wp-content/themes/twentythirteen/style.css
===================================================================
--- wp-content/themes/twentythirteen/style.css	(revision 23619)
+++ wp-content/themes/twentythirteen/style.css	(working copy)
@@ -738,7 +738,8 @@
 }
 
 .wp-caption .wp-caption-text,
-.entry-caption {
+.entry-caption,
+.gallery-caption {
 	color: #220e10;
 	font-size: 18px;
 	font-style: italic;
@@ -1244,6 +1245,8 @@
 .gallery-item {
 	float: left;
 	margin: 0 4px 4px 0;
+	overflow: hidden;
+	position: relative;
 }
 
 .single .gallery-columns-1.gallery-size-medium,
@@ -1328,14 +1331,22 @@
 }
 
 .gallery-caption {
-	color: #777;
-	font-size: 14px;
-	font-style: italic;
-	padding-top: 4px;
+	background-color: rgba(255, 255, 255, 0.8);
+	height: 50%;
+	line-height: 1.2;
+	margin: 0;
+	overflow: hidden;
+	padding: 2px 10px;
+	position: absolute;
+	bottom: -50%;
+	text-align: left;
+	-webkit-transition: bottom 400ms ease;
+	transition:         bottom 400ms ease;
+	width: 100%;
 }
 
-.gallery-caption {
-	width: 90%;
+.gallery-item:hover .gallery-caption {
+	bottom: 0;
 }